When I first started vintage racing my MGA coupe one of the other racers
suggested that I use Castrol motor oil for 4 stroke motorcycles in my
transmission. It is available at most, if not all, motorcycle shops. It is a
straight weight oil, SAE 30 to SAE 70, and has the necessary additives to
withstand extreme heat of an aircooled motor plus the pressure/ friction load
of transmission gears.
I have used SAE 40 in my MGA 1500 roadster and SAE 50 in my MGA 1600 coupe
since 1988 with no ill effects. I did tear down the coupe's tranny to replace
synchros but the bearings, etc. showed no wear after 6 years of racing. I do
thrash that tranny quite hard on the track.
The cost is about twice that of regular motor oil.
Gene "the older I get, the faster I was" Myszkowski
1959 MGA 1500 roadster on the road since 1962
1959 MGA 1600 coupe on the track since 1988
1968 MGC-GT soon to be on the road to INDY