My original manual calls for single-weight either SAE-20 or SAE-30, as multi-weight oils were just becoming popular in the 1950s. The second number is most important with multi-viscosity oil as that is what you have after start and warm-up. Even more important with these engines with primitive or no oil filtration and poor crankcase ventilation is regular changing of the oil every 1,000 -1,500 miles, or annually whichever comes first.
