[GTJIM]

The sources I quoted barely mentioned synthetic gearbox oil, just warned about GL-5 synthetic gear oil - but Redline MTL looks good and is GL-4 rated:

"This 70W80 GL-4 Gear Oil (SAE 5W30/10W30 engine oil viscosity) is designed for use in manual transmissions and transaxles. Provides excellent protection of gears and synchronizers and its balanced slipperiness provides a perfect coefficient of friction, allowing easier shifting. Popular in many BMW transmissions."

Probably one which Gil recommends ... and Bob certainly does!