That'll happen when stored,the oil will seep past the oil pump check ball and drain your tank. Your 91 is a bottom breather and if you still have the crankcase breather hose going to the air cleaner, disconnect it from the air cleaner into a catch can, crank the engine over and it'll puke up to a quart usually 1/2 a quart or so from the crankcase vent into the catch can the rest will return to the oil tank. The only way I know of to stop the oil from sumping into into the crankcase when being stored is to drain the oil tank or pinch the oil line, I'm not a fan of pinching off the oil line for extended periods tho.
Hope this helps you.