I remember when the S76 was first operated on the North Sea. The Allison C30 engine was prone to coking and turbine failures were a regular occurence.
Bond helicopters ran a trial to see which oil produced the lowest level of carbon deposits in the turbine bearings.
All six aircraft were operated on different oils, supplied in us quart sized cans and all operating from the same hangar, what fun.
Boeing uses Skydrol in the 737 hydraulic system but it uses Aeroshell 41 (OM 15 equivalent) in the flap gearbox's.
The possibilities for error are endless.
Many helicopters use the same oil in engines and transmissions, others have something different for transmissions. The B0 105 uses engine oil as a hydraulic fluid for the rotorbrake.