htown - yes, I see now - I had the transmission lube, not the gearbox.
Hillbilly bike - I found the MT-1 designation (https://www.oilspecifications.org/api_gos.php
) - it provides "protection against the combination of thermal degradation, component wear, and oil-seal deterioration, which is not provided by lubricants in current use meeting only the requirements of API GL-1, 4, or 5." So my understanding would be that I'd have to find a bronze-friendly oil and if it happened to be MT-1 that would be icing on the cake. Redline 75-90 and Brad Penn Classic - I'm taking notes.
Sam - GL-1 90W for tractors sounds promising. Gl-1 MT-1 would seem like the ideal ticket. There is a Tractor Supply chain store within striking distance so that's a possibility.
Dave - it was always my understanding, and I'm probably wrong, that GL4 was generally ok, but GL5 could be a problem. I always used 85W-90 GL-4 and never had a problem, but last summer I think I might have used some GL-5 that was lying around.
Anyway, I was surprised this winter when I drained the gearbox and then pulled off the outer cover after finding bronze in the oil. Go to this url and click on the fourth and fifth thumbnails from the top and you'll see what I found. When I pulled everything apart I found those little bronze flakes everywhere and in everything, but 90% of it settled between the inner and outer covers and was trapped there
Initially I thought I'd spun the drive side layshaft thrust washer, and it had moved (i'd driven the locating peg down) a little bit, but the only damage to it was a scrape on the backside, about a third of the way around - not enough it seemed to account for all the bronze pictured in the photos.
I measured the thickness of the thrust washers and they were both still in spec. I took high gear and layshaft gear with bronze bushings to a machinest and showed him the specs and he said they were ok. Was the gear oil I used unkind to the bronze in the gearbox? Or was it something else? (Probably something I did according to the odds).
Every gear oil I've looked at around here is GL-4 AND GL-5, and after replacing all the gearbox bearings after just one year I'm a little paranoid about suffering a repeat, and thus my heightened concern about using an appropriate product.
Ed from NJ - I have heard it said not to use hypoid oil, but it seems to work ok for you in your Brit iron.
I guess at the very least I'm going to look for a product that specifically states that it's ok for bronze. Not so easy to do online. My next move will definitely be to visit the Tractor Supply store.
Thanks to all posters for your input!