Gearbox oil

most would say stick to EP90 or 80 ( the GL4 version that does not attack the non-ferrous bushes). EP: extra pressure, to cope with high contact pressures between gear teeth meshing.

In my Model 50 with AMC gearbox, I use the same oil as I use in the engine: Morris C40. I know it's a racing oil, but I like the smell and it has proved to be a good oil, as the engine still doesn't really consume oil after years of use and the internals are in good order.

When I found myself short of Morris C40, I've used ordinary 10W40 for motorcycle engines. The latter is important, as oil for motorcycles is designed to handle the crushing effect of gearbox gears and is also designed not to let slip a wet clutch. Or so I'm told. That's why I alwayse use that oil in the oil bath to prevent clutch slip and it seems to work.

Anyway, to get back to your question: any modern motorcycle oil should do, as it's designed to cope with gears. I look at it as the WD40 of oils: you can use it for everything. The suggested EP80/90 in GL4 guise is even more designed for the job, so a sound advice. However, it might be overkill, just like me using the C40 in a road engine. But I like overkill.

Still, I've been using simple modern 10W40 in my Yamaha TRX for 200.000 km's and that engine still has virtually no wear to it (the bores were just honed at 100 k to give the new piston rings a chance of running in). The gearbox is still mint.

... the green stuff - was specifically designed to cope with the BMC Mini which had its gearbox in unit with the engine and hence shared oil between them. I can't remember the name of the NOC oil guru who sadly died some years ago but I seem to remember he recommended a 20W/50 for both engine and gearbox.

Take advantage of the preunit setup that these bikes use    i.e. seperate gearbox, chaincase, and engine oils. Means you can use a dedicated oil for the intended job. EP80/90 for the gearbox would be my choice.  In the case of the BMC Mini and its engine/g/box oil as one, in my 850 minivan that i had in the early eighties, after a long run, say to Cadwell, about 70 miles away the g/box/ gearchange action was very different to shorter journeys. Far looser or notchy. I put that down to the oil getting hot/thin. 20/50W oil will work in the g/box, would i use it? No, i'd use a pukka g/box oil.  

... our gearboxes rarely get anything more than warm so can't see any oil thinning to any extent. I tend to use whatever I have and happen to have most of a gallon of straight 40 so would probably use that in the gearbox.

I seem to remember that the late Fred Barlow suggested EP140 in the gearbox but suspect that was more for competition machines.

I'm not sure how many people have experienced gearbox trouble due to the "wrong" oil.

What ever you use My advice would be to change it regularly at least annually  the gearboxes are prone to condensation. 
Dan 

Dan is right about condensation  in the uk climate. EP will give the best protection from gear wear. Not tried 140 , might leak less , Curious  what the SAE number is ? about 50 ? . I suppose if you already have a lot of clutch and box  drag  ( i dont!)  a thinner oil would be usefull. Dont think a model 50 will stress the box that much so any  classic 40 will cope.

Many years ago I was having a chat with a Castrol rep' and the subject came round to motorcycle lubricants and what he would recommend for my G9 as I was using and still do Castrol GP 50.
He told me that as I was ordering some SAE 140 I could use that as it was the same product!
That 25ltr drum lasted me many years so long as I shook the drum vigorously before using it at each oil change.