Find answers to the most commonly asked questions about ExxonMobil aviation products and services.

For additional information, or to make specific inquiries, please contact your ExxonMobil aviation representative or submit an inquiry online.

General questions

There are many acronyms used in the aviation industry. What do they mean?+

Here is a brief list of common industry terminology:

APU Auxiliary Power Unit
CSD Constant Speed Drive
IDG Integrated Drive Generator
BPR Bypass Ratio
TBO Time Between Overhaul
MTBF Mean Time Between Failures
MTBR Mean Time Between Removals
SFC Specific Fuel Consumption
EGT Exhaust Gas Temperature
ETOPS Extended Twin-engine Operations
IFSD In-flight Shutdown

What do the dates on the certificate of analysis refer to?+

Dates defined:

  • Date Printed represents the date the certificate of oil analysis was printed
  • Plant Dispatch Date represents the fill date; this is the date the product was actually packaged/filled in its container
  • Date of Manufacture represents the certification date, or testing date

What date should I use to determine shelf life remaining?+

The shelf life on an aviation lubricant product is listed on the container/package as the “Use by” date. In the event that the package/container does not have a “Use by” date, check the manufacturing date list in the Certificate of Analysis (COA) and add the recommended shelf life. For information on recommended shelf life, please refer to the COA (shelf life referenced on bottom of the page), or the ExxonMobil Aviation Shelf Life Bulletin.

Jet turbine oils

What is a jet oil?+

Jet oils are lubricants developed specifically for aviation gas turbine engines. Jet oils are designed to operate over an extreme temperature range. Temperatures can range from -40 degrees Celsius for cold starting at high altitudes and go up to 250 C for bearing lubrication. Jet oils can also be used to lubricate certain land-based and marine engines (aviation derivatives) typically used for power generation or propulsion aboard seagoing vessels.

Are Mobil Jet oils compatible with elastomers, plastics, metals and seals?+

Mobil Jet oils are compatible with the materials provided under normal conditions of jet engine operation. For compounds and components (e.g., plastics) not typically used in aviation engines, compatibility studies must be run. Even with elastomers, great care must be exercised when applying data from one study to another. Unique circumstances could result in a response that is different from that predicted in a study. Elastomers, like lubricants, have many and varied formulations, all of which may not react in a similar fashion under the same conditions. Consequently, each situation/application must be evaluated separately.

What are the differences between a land-based turbine installation and those that operatie in the aviation environment?+

The biggest difference is that land based installations do not have the benefit of ram air cooling and therefore the reservoirs typically are larger (500+ gallons versus 6 gallons) in order to provide sufficient residence time for oil cooling. Additionally, many land-based turbines are run intermittently for peak electrical or gas compression periods, as opposed to every day as in the case of aircraft engines. All of this results in the potential for a higher possibility of acid formation due to buildup of moisture/water in the lubricant. As such, many equipment manufactures recommend reservoir drain and flush service intervals to ensure continued oil performance. It should be noted that while the larger reservoir systems provide for oil cooling, the internal engine temperatures are every bit as severe as those seen in an aircraft engine. Therefore the need for good thermal stability is still a critical performance feature of synthetic jet turbine lubricants used in land-based applications.

How do I change a system from an existing brand to a Mobil Jet oil?+

First, always consult with the engine/equipment builder recommendations prior to enacting the changeover to ensure compliance. Next, we recommend converting systems by simply "topping-off" the system with the new oil. This approach allows the original brand of oil to be phased out gradually over time as more "top-offs" occur. The standard "drain and fill" method is less desirable because there is the potential for disturbing system equilibrium (e.g., seal swell/compatibility, deposit loosening/washing, etc). Since oil is consumed in a jet engine on a regular basis (average - 0.20-0.80 quarts/hr), estimates indicate the oil system will typically contain more than 95 percent of the new oil in a few hundred hours of operation.

Are Mobil Jet oils compatible with competitive aviation gas turbine oils?+

Yes, they are compatible. The oil approval process includes compatibility testing for all jet oils. Approval requirements mandate that jet oils must have full capability in any proportion with any other similarly certified oil. The US Military also requires that qualified oils be compatible with other oils that meet the same specification.

Why is Mobil Jet Oil 254 lubricant darker in color?+

Gas turbine oil coloring is determined by the additive package selected and can vary between oils. It is not an indicator of oil quality or suitability.

How do we obtain a certificate of compliance for Mobil Jet oils?+

A certificate of compliance is a document similar to what's commonly referred to as a Certificate of Analysis (COA). COAs are filed for every batch of Mobil Jet oil produced. A COA can be obtained by contacting your ExxonMobil source point or downloading a copy from our website.

How should the Mobil Jet oils be handled+

With regard to handling, please refer to the Material Safety Data Sheet (MSDS). Generally no special precautions are necessary beyond standard good hygiene practices. In addition, there is no need to shake, stir or otherwise agitate the oil before use. The ester base oils used in Mobil Jet™ Oil II and Mobil Jet™ Oil 254 gas turbine lubricants have excellent additive compatibility and long-term storage stability characteristics.

What is the shelf life for Mobil Jet oil products+

The shelf life for all Mobil Jet oils in metal quarts is 10 years, and five years for all products in pails or drums. We recommend effective inventory management be employed to ensure the stock of Mobil Jet oils is rotated for storage and use.

Why are the Mobil Jet oils sold in metal quart cans?+

Many airlines prefer quart cans. Quart cans provide for ease in handling when performing reservoir "top-offs" during maintenance checks. Additionally, quart cans have a longer shelf life as a result of hermetic sealing. Jet oils are extremely hygroscopic due to their chemical composition; they can attract/pick up moisture from the air easily, even past the bungholes on pails and drums, due to expansion and contraction associated with temperature swings. Once a quart can is opened it is difficult to reseal, and as such, should be used entirely or disposed of. Reusing open quart cans increases risk associated with collection of moisture and/or other potential contaminants (e.g., dust, dirt, etc.), therefore it is recommended for single use application only.

Are jet oils hazardous or toxic?+

Mobil Jet oils have a safe history of use in both ground and air operations for more than 50 years. ExxonMobil is not aware of any adverse long-term health effects associated with aircraft use of jet engine oils. Refer to the MSDS (Material Safety Data Sheets) for additional handling and safety information.

Hydraulic fluids

What is a hydraulic fluid and how is it used?+

Hydraulic systems use a confined fluid to transmit power. The fluid used to transmit power is called a hydraulic fluid, or oil, and also functions to cool and lubricate the hydraulic system. Aviation hydraulic systems fulfill various functions, including moving flight control surfaces that help steer, take off and land aircraft. Hydraulic fluids help to engage aircraft brakes and deploy or retract landing gear.

Are Mobil™ HyJet™ fluids compatible with other hydraulic fluids, elastomers, plastics, metals and seals?+

Mobil HyJet products are fully compatible in any proportion with each other and with other commercially available phosphate ester based hydraulic fluids. Additionally, aircraft OEM require a multitude of compatibility tests, including compatibility tests with metals, elastomers, solvents and paints. Please contact your ExxonMobil aviation representative for specific compatibility inquiries.

Are there any special handling requirements for Mobil HyJet products?+

In addition to normal housekeeping requirements used to minimize contamination, you should also minimize any periods of exposure of the product to air by keeping containers closed for storage. Mobil HyJet products will rapidly absorb moisture from the air, which will lead to hydrolysis – a chemical degradation of the product. Previously opened containers with low fluid content should therefore not be stored for long periods of time.

Long-term exposure to Mobil HyJet products can cause irritation and dry skin. These fluids will also be painful if they come in contact with the eye, although there is no evidence of long-term harm to the eye. Accordingly, proper personal protective equipment should be used when handling HyJet products.

Proper handling of Mobil HyJet products is well documented in the MSDSar.

Why are Mobil™ HyJet™ products purple?+

Fire-resistant hydraulic oils are dyed purple to differentiate them from 5606 fluids, which are red in color.

How are Mobil™ HyJet™ IV-Aplus and Mobil™ HyJet™ V hydraulic fluids different from conventional hydraulic oils (MIL-PRF-5606)?+

Mobil HyJet IV-Aplus and Mobil HyJet V are fire-resistant hydraulic oils while other 5606 fluids are not considered fire-resistant. Fire-resistant fluids have evolved over time as commercial jet travel has become more prevalent. Fire-resistant fluids were developed to accommodate increased aircraft speeds during landing and withstand higher braking temperatures. Fire resistance properties of these oils include high auto-ignition temperature and zero flame propagation. Additionally, the oils must meet special OEM test requirements such as hot manifold ignition and high- and low-pressure spray tests, among others.

What is the shelf life for Mobil™ HyJet™ IV-Aplus and Mobil™ HyJet™ V hydraulic fluids?+

The shelf life of Mobil HyJet IV-Aplus and Mobil HyJet V hydraulic fluids is 10 years for quarts and gallons, and five years in pails and drums. The date of manufacture can be determined using the batch and fill codes printed on each package.

How do I convert an aircraft hydraulic system from a competitive product to a Mobil™ HyJet™ product?+

Conversion is typically completed using the “top-off” method in which Mobil HyJet products are added to hydraulic reservoirs as required to maintain proper fluid levels. Using this method, the original fluid will gradually be phased out as more “top-offs” occur. Please contact your ExxonMobil aviation representative for specific conversion suggestions.

Do you have a hydraulic fluid analysis program?+

Yes, ExxonMobil aviation offers Exxcare as a routine in-service fluid analysis program for Mobil HyJet customers.

What is the difference between Mobil™ HyJet™ IV-Aplus and Mobil™ HyJet™ V hydraulic fluids?+

Mobil HyJet V hydraulic fluid offers extended fluid life over Mobil HyJet IV-Aplus without sacrificing fluid performance in preventing wear, rust and deposits or increasing weight. Mobil HyJet V hydraulic fluid is also certified for use in 5,000 psi hydraulic systems.

Why is ExxonMobil rebranding Exxon HyJet™ to Mobil HyJet?+

The Mobil™ brand carries significant brand equity and is widely acknowledged as a premiere brand and leading supplier of high-performance lubricants. With the rebrand, our former Exxon products now align with and leverage the strength of the Mobil brand.

What exactly is changing with the rebrand?+

There will be no change in product formulation. The changes taking place are related to brand elements and GHS labeling requirements. Changes include:

  • The product brand will be changed from Exxon to Mobil. Exxon HyJet™ V will be rebranded to Mobil HyJet V. Exxon HyJet™ IV-Aplus will be rebranded to Mobil HyJet IV-Aplus.
  • The Running Tiger icon will be replaced with the Mobil Pegasus.
  • Quart and gallon size cartons will be harmonized to use a white Mobil branded carton. We are discontinuing the use of craft/brown cartons, which some countries and customers may have seen previously.
  • Warning labels are being added to the face of the packaging where applicable (Global Harmonized System (GHS) and the U.S. Occupational Safety and Health Administration (OSHA)).

Greases and gear oils

What makes aviation greases different than other types of lubrication grease (e.g., bearing or multipurpose)?+

No content

If grease incompatibility occurs, what are some initial effects that might be noticed?+

When incompatible greases are mixed together, the consistency typically becomes harder or softer than the individual greases. These consistency changes generally become more pronounced as operating temperatures and/or the rate of shearing increases.

While in operation, incompatible greases may also exhibit excessive oil separation or bleeding tendencies.

What happens when greases oxidize?+

Grease oxidation is not a well-defined occurrence and can take many forms, including consistency change (harder or softer), color change, odor generation, increase in noise from the component or a temperature increase of the component.

The most common methods to screen for grease oxidation are vibration testing or thermal imaging of the component. An increase in either could be a sign of the grease oxidizing.

How long does grease actually last? What's the average shelf life for aviation greases?+

Shelf life specifications can differ between aviation greases. While the average industry shelf life of aviation greases is about three years, ExxonMobil aviation greases offer extended shelf life beyond the industry average, ranging from six to 10 years – more than double the industry standard.

ExxonMobil aviation grease shelf life is listed as the “use by” date on the container. To maximize shelf life, normal storage temperatures should range from 0°C to 40°C (32°F to 104°F), and containers should be kept indoors in a clean and dry environment to avoid hydrolysis and other environmental factors.

Mobilgrease™ 33 and Mobilgrease™ 28 aviation airframe greases have a shelf life of 10 years, more than three times the industry standard.

Mobil™ Aviation Grease SHC 100 has a shelf life of six years – double the industry standard.

Where/how should you store greases?+

Grease containers should be stored indoors in dry, cool and clean environments. Normal storage temperatures should range from 0°C to 40°C (32°F to 104°F). In the event that a stored grease is briefly exposed to severe temperatures or environmental conditions, technicians should consult their suppliers with concerns.

Once a container is opened, the grease should be used as soon as possible to avoid potential contamination. Since the rate of grease degradation can be impacted by exposure to contamination and storage and handling conditions, the listed shelf life recommendation for a grease is no longer applicable once a container is opened.

For more information on storage and handling best practices, review our technical topic here.

What does the viscosity change do when you go from an ambient temperature to an engine operating temperature?+

As the application temperature increases, the viscosity of a grease’s base oil will decrease.

When selecting greases, operators should also consider the base oil’s viscosity index, which measures an oil’s change in viscosity at different temperatures. Typically, greases formulated with synthetic base oils have a higher viscosity index compared to mineral base oils and will experience less base oil viscosity loss at higher temperatures.

How do ExxonMobil aviation greases help operators’ sustainability initiatives?+

As a result of the 10-year shelf life associated with Mobilgrease™ 33 and Mobilgrease™ 28, operators can reduce waste disposal, carrying costs and inventory turnover.

Extensive testing and research conclude that ExxonMobil aviation greases are capable of delivering the same quality of benefits throughout their full lifespan. According to this quality assessment, operators can safely rely on these greases to potentially reduce costs and continue to support their sustainability efforts for extended periods of time.

Mobil™ Aviation Grease SHC 100 carries a six-year shelf life, which is more than double the shelf life of similar greases. In addition, Mobilgrease 28 is approved for use as both a multipurpose airframe and wheel bearing grease, making it useful for operators looking to consolidate grease inventories, reduce waste and lower operating costs.

What happens to greases when exposed to water?+

We recommend that exposure to high water concentrations, including precipitation and de-icing fluids on the tarmac, be avoided. Potential issues could involve hardening or softening of the greases, increased corrosion or decreased effectiveness of the performance additives. For greases formulated with ester base stocks, you may see increased hydrolysis or breakdown of the grease. To be safe, we recommend that technicians should evaluate their grease every 10,000 hours for irregularities and consult their supplier with any questions. Hydrolysis is preventable.

What are the effects of using contaminated greases (e.g., corrosion, pitting, etc.)?+

The use of contaminated greases can cause a wide range of issues, including deposit formation, reduced wear protection, increased wear, increased corrosion and reduced bearing life. We recommend that if a contaminated grease is used, the grease must be replaced immediately and the part should be examined for potential damage.

What causes a grease to change colors? Can discoloration affect the grease?+

Changes in grease color can be the result of a number of factors, some of them more negative than others. For instance, the change can merely be caused by a change in the chemical dye used in the grease.

However, if the color change is caused by high temperatures, contamination or reactions with other substances, the color change may indicate a change in grease performance. Some of these effects include:

  • Chemical changes in a grease’s base oils or additives, such as hydrolysis, which may reduce the load-carrying or structural stability of the grease

  • Presence of particle contamination that can cause premature wear of bearings or affect grease quality

  • Reaction with unknown material may cause a change in color

What happens when you find black grease and not green grease during an inspection?+

Grease color changes in-service can be a result of operating temperatures well above a grease’s temperature rating, or a result of contamination (e.g., dust from carbon brake that causes wheel bearing grease to appear dark).

In some airframe applications, a black grease may signify hydrolysis of the ester base oil or grease, identified with some BMS3-33 and MIL-PRF-23827 approved greases.

Are all greases compatible with one another?+

No, not all greases are compatible, and the mixing of grease brands should not be taken lightly. If commingled greases are incompatible, the grease intermixing can result in reduced performance. All greases contain base oils, thickening agents and chemical additives that can interact adversely if incompatible greases are mixed.

To confirm if grease formulations are compatible, operators can consult the Standard ASTM D6185– Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases, which is based on performance data from testing mixtures of both greases.

Most OEM, industry and supplier recommendations suggest performing a compatibility test to assess the risk of mixing two greases. Standard compatibility tests may consider the stability of the oil/thickener matrix by evaluating a grease’s shear stability, dropping point, storage stability and high-temperature performance.

ExxonMobil aviation has tested many combinations of greases to find that compatibility is generally “borderline.” The normal aviation industry practice is to fully purge all old greases from any application, regardless of compatibility. This is a standard operating practice and should be followed without exception.

For more information on grease compatibility and for conversion guidance, please review the technical topic here.

How do the base oil properties vary and what is the impact on the grease?+

Base oils can consist of a wide range of fluids from low-viscosity mineral oils to heavy cylinder stocks to specialized synthetic lubricants. The inherent features seen in a base oil may also be seen in the grease, such as good high-temperature performance, oxidation resistance or low-temperature performance. Base oil selection is generally determined on the desired performance features of the grease.

What causes grease to produce an odor? Does grease odor impact performance?+

While many base oils and additives used in greases have a defined odor when fresh, odors can also be the result of in-service degraded grease. However, ExxonMobil aviation greases formulated from 100 percent polyalphaolefin (PAO) base oil, such as Mobilgrease™ 33, will minimize these objectionable odors. Greases containing an ester base stock may be susceptible to hydrolysis and could generate an odor.

Grease odor does not impact grease performance.

Is it normal for greases to exhibit some oil “bleed” or separation?+

Depending on the storage conditions, duration and temperature, greases can exhibit some degree of oil separation. For example, higher oil separation in a grease can merely be a direct effect of formulating it with a lower-viscosity base oil.

If oil is collecting around the dome or crevices at the top of a grease pail, the oil can be remixed into the grease. If, however, the pail’s surface is completely covered with a layer of separated oil, then operators should contact their supplier before applying the grease.

Again, operators should be wary of instances where more than one type of grease is used to ensure that grease transfer does not cause incompatible greases to come in contact with each other. Incompatible greases may result in an increase in oil bleed.

For more information on oil bleed and grease separation, review the technical topic here.

Is there anything we can do to minimize oil “bleed” or separation?+

You can help minimize oil separation by storing oils in a climate-controlled environment and releasing pressure on the pumping devices used to apply the grease when not in use.

What is oil bleed and how does it impact the grease?+

Oil bleed is a term used to explain the separation of oil or seepage of oil from the grease during normal operating or storage conditions. Oil bleed is easily identified by the presence of oily sections of greased components and/or the formation of small pools of oil around componentry.

Excessive oil bleed can impact a grease by causing it to harden, rendering it unsuitable for use.

What temperature ranges are appropriate for ExxonMobil aviation greases?+

ExxonMobil aviation greases are all expertly formulated to deliver exceptional performance and component protection even under extreme operating conditions. Each of our aviation greases delivers exceptional performance over wide temperature ranges:

  • Mobilgrease™ 28 – A multipurpose airframe and wheel bearing grease for plain and rolling bearings with an operating temperature range of -54ºC to 177ºC (-65ºF to 350ºF)

  • Mobilgrease™ 33 – A high-performance, BMS3-33-approved multipurpose airframe grease with an operating temperature range of -73 ºC to 121ºC (–100ºF to 250ºF)

  • Mobil™ Aviation Grease SHC 100 – An advanced wheel bearing grease with an operating temperature range of -54ºC to 177ºC (-65ºF to 350ºF)

What is flash point and why is it important to grease?+

Flash point is the lowest temperature at which a grease gives off sufficient vapors that will ignite in air when exposed to an ignition source. The flash point of a grease is often governed by the flash point of the oil used in the grease.

What happens if I use too much grease?+

Over-greasing of a bearing or component can lead to a number of issues, including high operating temperatures, seal failures and bearing failures.

How do you know a grease needs to be replaced?+

The most common visual indicators to help you determine if a grease needs to be replaced are to look for any hardening, softening or significant color changes. More advanced techniques involve vibration analysis, used grease analysis or thermal imaging. A detailed lubes program analysis by your supplier will help you identify the key features you need to assess grease life in service.

How do you clean grease off an application/bearing when changing grease or making a grease conversion?+

A number of options exist for removing grease from your application or bearing. These can range from purging and wiping down the part to solvent washing of the part. It is generally recommended that you contact your OEM supplier to determine what is an acceptable method to clean the application or bearing.

What does the color of grease mean? Why does aviation have red, green, blue and tan colored greases?+

The color of fresh grease is simply a function of dyes added to the formulation, which is often required by aviation industry organizations such as SAE International Aerospace or by an OEM QPL-specific directive. Other greases are required by specifications to be free of dyes.

Different grease colors sometimes help to avoid accidentally mixing greases; however, greases with the same color are not always compatible. For more information on aviation grease color, please review the technical topic in full here.

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