I'm not aware of any oil lines to or from the expansion tank. No, the sludge in your oil wouldn't necessarily be blue. Unfortunately an overheat can be very damaging to an aluminum engine such as the M54. And you were almost dry when it happened. I think I'd have it checked out by a good Indy. Other than the possible head gasket you also have some serious coolant loss.

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2001 3.0i * SOLD *
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2010 E83 Alpine White