Same thing almost happened to me.
My 300SE drain plug would not
budge even after the liquid wrench.
I decided not to tempt fate and just
took it in for an oil change. No problem.
More arm pull space at a shop.
Any way I had thought ahead
of the option of using a pump device down the
dipstick tube to get the oil out.
Search the archives for the name of this
device.
Good luck

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1979 300D
133K miles
1989 300SE
1995 C280