This is what we have used for some years the fully synthetic in race boxes as they get very hot and semi synthetic in road boxes it has a bit more body than ATF
it is a light gear oil, viscosity about the same as a 30 engine oil but with all the advantages of a gear oil make sure you use a GL4 grade as they have no sulphur additives that could affect any brass/bronze components.
Terry
Terry put me on to the 75-90 semi synthetic oil a year or so ago. Now have it in all my gearboxes and I'm a convert. It even smells like a gear oil should!
I have dug various books on tribology out of libraries over the years. Some suggest that 20 is marginally thin for heavily loaded manual gearboxes ie towing caravans out of the Grand Canyon. Some sources suggest that the older mutigrades are seen by gears as the thinner. But with Seven powers, loadings must be much reduced cf moderns.
Abrasive wear is now largely countered by anti wear additives. The tendency to pit is probably increased with thin oil but few Sevens are going to do another 150,000 miles.
For synchro Sevens noise and leakage likely the main determining factors originally.
