A piece of foil that size may even stay up in the top of the engine. With the fill at the front and the drain at the rear with all the valve springs and rockers in between, it may get hung up there. If it doesn't come out in the draining of the oil, my next look would be to pull the right side valve cover. So much easier to put on than the oil pan, it may be worth a look. Good luck.

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Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)