I had the same problem with my '86 300E, and I had been the one to change the oil on numerous occassions previous, so I have no idea how it got so seized. At any rate, I agree with Arthur....those tools from Sears work great! .....albiet a tad expensive just to get out one bolt! But....I know this might seem obvious, but make sure you have the replacement drain plug before you extract the old one!

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold