Once upon a time in the dim and distant past I was gainfully employed in the buying dept of a local engineering firm, part of my job being to see visiting reps. At the time I was running a vintage bike (not BSA) as well as my Shooting Star, and multi grade oils had not long appeared on the market. (Told you it was a long time ago!) A pal had just changed from Castrol Grand Prix sae 50 to a certain brand of green 20-50, leading to the almost immediate and spectacular demise of his Star Twin's bottom half. This led to shall we say some thought, and when a suitable Oil Company rep appeared, I proceeded to pick his brains. His advice was thus; "Multi-grade oil, say 20-50, is essentially a 20 grade oil with things called VI improvers added. These stop the oil thinning out as much as it would otherwise. Unfortunately, due to the temperatures and pressures in these old engines, the VI improvers rapidly break down, leaving you with basically an SAE20 oil, which the clearances in these old engines are unable to tolerate." His advice was to use a monograde oil, for preferance one formulated for diesel engines, as these operate at much higher loadings than a petrol engine. This I proceeded to do with my A7SS, on which I subsequently covered 90,000 miles in 6 years. The vintage bike got fed something a bit cheaper, being a total loss system. As a matter of interest, I found out some years later that Volkswagen and Porsche made the same recommendation for their air cooled engines. Might answer queries as to SRM's oil spec.