Cause of oil leak?

[Allison Bloodsworth <Allison.Bloodsworth@xxxxxxxxxxx>]

I own a 1991 E30 325Ic. The other evening I was on the interstate and
   noticed smoke from the rear of the car. I pulled off at the
next   exit and the car died. The back was covered in what looked like
oil. I   had the car towed to my house. I cleaned off the engine and
started it   again in the daylight. It ran very rough and wanted to die
on me. Very   quickly it began smoking again. The source of the oil (as
best as I could   tell) was from the seam in between the block and the
head about at the   point that is between the two sets of headers. The
oil was leaking out   and falling directly on the exhaust. This may seem
like a stupid   question, but having never had this happen before, my
first guess is that   I need to replace the head gasket. Am I right? How
can I know for sure?   Might there be other problems? I ask because
someone suggested that the   piston might have "welded to the block"(?)
and another suggested a   cracked block might need replacing. Any help
is most appreciated. My   email is <derekbloodsworthTAKEOUT@domain.elided>
without the TAKEOUT   (obviously).

   Derek

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