Originally Posted by
riff_raff
In the US, most newer rotorcraft gearboxes use a dedicated transmission oil like DOD-PRF-85734 or a commercial equivalent, rather than turbine engine oils like MIL-PRF-7808. DOD-PRF-85734 transmission oil is a synthetic base lubricant that is hard on some materials and finishes. So one consider with using this oil is ensuring all elastomer seals and coatings are chemically compatible.
Would that include the Sikorsky S-92A tailrotor drive I wonder?