3-1. General. The oil system supplies oil to the aircraft engine for lubrication. Oil systems will be serviced with the
specification and grade of oil listed in the specific aircraft maintenance publication. Do not allow oil to become
contaminated through careless handling, use of dirty oil containers or equipment, or entry of foreign material through
openings of the crankcase. Identification, contamination, and general maintenance practices will be covered in this
3-2. Safety Precautions and Procedures. The following warnings and caution will be followed when working with
aircraft oils and oil systems.
Lubricating oil, MIL-L-7808 or MIL-L-23699, contains an additive which is poisonous and
absorbed readily through the skin. Do not allow oil to remain on skin any longer than necessary.
Oil is a skin irritant. If oil gets on skin, wash thoroughly. If oil soaks into clothes, change clothes
Oil gives off fumes that can cause injury to personnel. Use oil in a well-ventilated area.
Oil can soften paint. If oil drips on painted surface of aircraft, wipe up immediately with clean
3-3. Types of Oils. Mineral fluid lubricants are used as the principal lubricant in all types of internal-combustion
engines. They can be pumped easily and sprayed readily. They also dissipate heat quickly and provide a good
cushioning effect. Synthetic lubricants are used in turbine engines because of the high temperatures that are developed.
Synthetic lubricants are not made from natural crude oils which would breakdown under high temperatures.
3-4. Identification of Oils. Identification of oils for both reciprocating and turbine engines are explained in the following
Reciprocating Engines. Standard oil for reciprocating engines are specified in table 3-1.
Turbine Engines. Turbine engines require oils which have a lower pour point, or better viscosity temperature
characteristics, and a higher degree of resistance to oxidation or thermal decomposition than petroleum derived oils.
The oils in table 3-2 are the synthetic oils which meet these requirements and are commonly used by turbine engines.
Turbine engine oil MIL-L-23699. This oil has a greater life than MIL-L-7808 and has generally replaced
MIL-L-7808 in the temperature ranges shown in figure 3-1. This is recommended oil for all new and overhauled turbine
Turbine engine oil MIL-L-7808. This oil is used in extreme low temperature ranges, as shown in figure 3-1.
Color identification. There is no easy way to look at oil and tell the difference between the two types of oil.
The only way to determine which type of oil is in an engine is by careful maintenance of servicing records.
Compatibility. MIL-L-7808 and MIL-L23699 oils are required by specifications to be compatible with each
other. However, adding MIL-L-7808 oil to a system which has been converted to MIL-L-23699 should be avoided, if
possible, since the addition of MIL-L-7808 oil will lower the concentration of MIL-L-23699 oil and thus will tend to nullify
the benefits derived from the MIL-L-23699 oil. In addition, indiscriminate mixing of the two oils may result in result in
pressures and flow limits different from the published values for either oil. When operating conditions necessitate
reservicing with MIL-L-7808, the system or components should be drained and reserviced as soon as MIL-L-23699 oil is
3-5. Oil Contamination. Oil is exposed to many harmful substances which reduce the ability of the oil to protect
moving parts. The main contaminants are gasoline, moisture, acids, dirt, carbon, and metallic particles. Because of
these contaminants, the oil must be changed at regular intervals according to the applicable maintenance manuals.