[rotorwrench]

I use SAE-50 grade 100 aircraft engine oil (AeroShell W100) with NO Lycoming additive. I could use the blue Swepco 201 SAE-90 that is used in the Robinson helicopter transmissions but the AeroShell W100 is less expensive and works fine for my 1951 Mercury cars.

SAE-50 motor oil is the same viscosity as SAE-90 gear oil due to the different way the two are graded.