ON 1971 models the engine oil and primary oil are the same. Oil moves in to primary from crankcase pressure, then any overflow drains back in to the case. If you drain the primary you should put a few ounces of oil in just to get things started, after that it is self levelling. Do not put ATF or other trans lubricants in the primary as it will eventually end up in the engine. I use 20/50 in most of my bikes.