If memory serves me 1991 Sportsters have oil tank check valve problem that allows oil to drain into crankcase.
Ding, ding, ding. I think we have a winner!
Jerry
Edit: I rang the bell too soon. After posting I had second thoughts and checked out the Sporty oil pump and tank. The Big Twin Evo had a check valve in the oil pump body, the Sportster for some reason did not. The only thing in a Sporty slowing down the oil as it gravity drains into the sump is the pump itself. If the pump is worn, I assume it doesn't slow the oil down much at all. On a new bike of that era it would usually take a couple months or more to drain the oil tank into the sump. Another fine piece of engineering from the MoCo. They really should have abandoned the entire dry sump design a long time ago.