BSA m20 gearbox oil should be a quality motor oil. Avoid heavy oils and gear oils like 80/90. Change oile more often than recomended eg at least once a year or when changing motor oil. You may have fun getting the oil out of a WLA as the original HArley box does not have a drain plug and the frame probably obstruct access to the BSA box drain plug. I have both a M20 and a WLA. To get the oil out of WLA lay the bike on it's side and let the oil drain out the filler. You can fit a drain plug to the BSA box in another spot in the WLA.
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