Hi mattsuzie,
here is a picture of the 420 drain plug. If memory serves me right, it is a 13 or 14 mm plug.
I doubt it will come through that drain as Murphy's law just won't let it happen. To take the oil pan off is not too difficult, there are only 2 bolts that go through the pan that have nuts on top as they hold some tubing from the transmission going to the cooler. Driverside. You need to take those 2 nuts off first, otherwise the bolt might brake off. I know from experience.
The hard part is to not overtighten the bolts upon installation. You need a new gasket (about $16?) and a torque wrench in inch pounds rather than footpounds. It needs only 8 ftlbs and my torquewrench was not sensitive enough. You can see the split gasket in the picture. The pan holes will get bent and you have a leak as a result of overtightening.
You will need 8 liters of oil.
Good luck
Reinhard Kreutzer

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This shows the oil pickup filter in the pan, that the foil might clog with the resultant oil pressure drop as moedip explained.

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Gravity--possible, but it would have to get past the 8 valve springs and rocker arms first. Oil pan isn't bad to remove, but the 4 bolts n the valve cover are easy and the gasket can be reused. I'd look there before even draining the oil. Not that draining (and changing) the oil is a bad thing.

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