During the early stages of turbine engine development, mineral oils were used to lubricate bearings, gears and other components within the engine.
As jet engines evolved through newer technology, these mineral oils were unable to cope with the increasing demands caused by higher thrust and compression, showing a lack of stability and thermal degradation over time.
Synthetic oils were introduced in the 1940’s to cope with this change, but jet engine development continued and higher temperatures were reached during operation.
This led to further advancements in jet oil, and ultimately a high thermal stability specification – MIL-PRF-23699 Type HTS.