In the early 80's Hughes 500D's were delivered with Mobil Jet II. In the UK AS 500 was much used and easy to obtain. We noticed that aircraft that were changed to AS500 had much higher levels of carbon forming in the oil. One of the aircraft had a turbine seize due to blocked oilways and all owners were advised to change back to Jet II.
The levels of carbon soon reduced to normal.
Around this time we also had AS555 which may have had better coking characteristics
but ate helicopter gearbox seals for breakfast.
Bond helicopters had massive coking problems with the Allison C30 in the S76. The problem was so bad that Sikorsky brought out a mod to armour plate the tail rotor drive shaft (20 kilos of steel) just in case the turbine bearings seized and the turbines departed. Bond were stripping and cleaning out a pair of turbines every weekend, in effect a six week cycle as they had six aircraft, in an attempt to alleviate the problem.
They ran a comparison test with each aircraft on a different engine oil, AS 500 was junked and Mobil 254 was found to give the best results. This was still the standard UK engine oil (used in the Arriel) for the company now CHC more than thirty years later.
Replaced with BP2380 for the PT6.