TURBINE ENGINE LUBE OIL
Lubricating oil, Military Speciication MIL-PRF-7808 or MIL-PRF-23699, contains an additive which
is poisonous and absorbed readily through the skin. Do not allow oil to remain on skin longer than
To avoid contamination, do not use previously opened cans of lubricating oil. A new sealed can of
oil must be opened and used.
a. 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 lubricants. MIL-
PRF-23699 oil and MIL-PRF-7808 oil are synthetic oils that meet these requirements and perform satisfactorily under
conditions which are too severe for petroleum products.
b. MIL-PRF-23699 oil and MIL-PRF-7808 are the oils used in the engines of turbine engine powered aircraft
as indicated in Table 3-1. These oils are compatible and can be mixed. However, it is recommended these oils not
be mixed as a normal practice.
Gear and transmission lubricants and their speciic application are listed in Table 3-1.
a. Adding MIL-PRF-7808 oil to MlL-PRF-23699 oil will reduce the MIL-PRF-23699 oil high temperature and
gear load carrying capability to the MIL-PRF-7808 oil capability.
b. Adding MIL-PRF-23699 oil to MIL-PRF-7808 oil which is used at very low ambient temperatures will in-
crease the oil viscosity which may cause oil starvation at cold temperatures (-40F, -40C or colder).
3-5. CONVERSION TO MIL-PRF-23699. For most turbine engine powered aircraft components, MIL-PRF-23699
has replaced MIL-PRF-7808 oil at temperatures above -25 F (-32 C). To reduce potential problems and provide
for a more orderly conversion to MIL-PRF-23699 oil, the following is recommended: Use MIL-PRF-23699 oil in all
new and newly overhauled turbine engines.
3-6. COMPATIBILITY OF TURBINE OILS. MIL-PRF-7808 and MIL-PRF-23699 oils are required by speciications
to be compatible with each other. However, adding MIL-PRF-7808 oil to a system which has been converted to
MIL-PRF-23699 should be avoided, if possible, since the addition of MIL-PRF-7808 oil will lower the concentration
of MIL-PRF-23699 oil and thus will tend to nullify the beneits derived from the MIL-PRF-23699 oil. In addition,
indiscriminate mixing of the two oils may result in pressures and low limits different from the published values for
either oil. When operating conditions necessitate reservicing with MIL-PRF-7808 the system or components should
be drained and reserviced as soon as MIL-PRF-23699 oil is available.
During the initial transitioning period some residual MIL-PRF-7808 oil will remain in the engine oil systems and some
degradation of the MIL-PRF-23699 is considered acceptable in order to eliminate costly and time consuming lushing
3-7. COLOR OF MIL-PRF-23699 OILS. The approved MIL-PRF-23699 oils vary in color depending on the sup-
plier. The color of these oils is due in part to the additives used by different manufacturers. Some of the additives
used may turn the oil a dark color or in some cases give the oil a reddish appearance. Therefore, the color of the
oil should not be used as the basis for deciding if the mechanical condition of engines or components is suspect.
In the past, color has been used to detect hydraulic luid contamination of the oil in an engine. However, chemical
methods are available for this purpose and should be used if necessary.