Synthetic Gear Oil
It makes sense to change the tranny fluid and Gear oil to synthetic, since they will last a lot longer than motor oil, and will not decompose due to the excessive heat of the engine.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"