Years ago when I was an apprentice, I remember seeing a Piper Lance return to our hangar with a **** load of oil on the windscreen/airframe as a result of the filler cap/oil dipstick not being secure. Luckily the pilot worked out what was going on and he was able to safely land. The amount of oil lost wast not significant enough to damage the engine. All we done from memory way to drain and replenish the oil and wash the engine and airframe.