Even with the oil drained, it will be tough to get a decent weld. Any spec of oil left will seep into the weld puddle and contaminate it. If you want it to hold, you had better drop the pan and clean front and back with a solvent (acetone is my preference). If you want to be cheap and lazy, leave the pan installed and drain the oil, rough up the leak with sand paper, clean with acetone, and gob some JB Weld on there. Wait 24 hrs and fill the oil back up. It might just work. If it fails, chip it off and get it properly welded.
