I use 75W 90 agtrans because I got a 1/2 a 44 of it at an auction when I was bidding on the drum pump.
Those who ride generally use Penrite gear oil or strait 40W oil when you can get it,
Those with trailer queens use semi fluid grease.
You can get away with it just so long as you don't ride the bike much and definately no longer than a hour .
I use straight 40 engine oil as recommended by BSA and always have...I also fit the sealed bearing (with the inner seal removed) as a matter of course during gearbox rebuilds...
That's about it and it has served well over a number of decades, multiple bikes and a lot of use...Ian
Thanks for these replies.
Trevor's right re opinions differing about suitable oils Down Under. Various authorities have recommended everything from semi fluid grease to 90w, never a mention of BSA's instructions to use engine oil. BSA probably knew something about it.
I use Penrite & oscillate (vacillate?) between 90 & 250. 250 in summer & 90 in winter.
I fill the box til it's about half an inch below the lip of the filler, should I put more in?
I have owned a WM20 for 38 years and always used SAE50 in the engine and gearbox, as shown in the army hand book, 50HD. Another army designation for this oil was M220, which again was SAE50. What some people do not realise is that SAE viscosities for gear and engine oils run on different scales and 50 engine oil is actually about the same viscosity as 90 gear oil, but without any of the additives that might harm any bronze bushes. Also for maintaining the bikes it made sense to use one type of oil.