Originally Posted by
RVDT
In years past many manufacturers used turbine oils in their main gearboxes. Mostly that has all changed as it worked but not that well in hindsight.
There are much better options available today. More common these days are synthetic industrial or agricultural lubricants and in a lot of cases automatic transmission fluids.
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.