A mechanic warned me against trying to remove the engine
block drain plug on my '91 300D 2.5 in order to drain and
flush the cooling system.  He said that because the plug
is aluminum and the block is steel, electrolysis will
likely have resulted by now and I'd probably just shear
the head off the plug with my wrench.

His advice was to simply drain and refill the radiator
several times with water instead, running the engine up
to operating temperature and then draining the water
before ultimately pouring in the coolant mixture.

I figured some penetrating oil and a few taps straight
down on the top of the plug head with a hammer ought to
loosen it adequately, since I've never had any trouble
with that technique on my W123s.  Any advice or caveats
about this?

Thanks,
Geoff