OK, so, after all this, I have more questions.
I took it into my Stihl shop and they don't have the right plate and bushings to pull the case apart (it was a blown crankcase gasket, or appears to be, as blsnelling predicted).
So, they can't easily pull it apart, and the parts they need to do so would apparently have to be ordered from the factory and would cost an arm and a leg. However, the mechanic (who used to be an aircraft mechanic, so I give him some credit) suggested that we might try cleaning the inside of the case out really well and trying some sloshing compound on it. I've been thinking about this, and I am almost game to try it. After all, if it doesn't work, I can still always crack the case and replace the gasket anyway - don't think an inner layer of sloshing compound would hurt that.
Does anyone have any thoughts on this?
Do I have other options I should consider? (For example, does someone have a spare set of the parts I would need to have them crack the case for cheap(ish)?) Should I send it to one of you fine fellows to fix for me? I got a hell of a deal on this saw (bought it as a parts saw, initially, but it turned out to be in far too good a shape to use that way) so I have some wiggle room and would like to fix it.
Is there any reason (and I'm thinking heat, here) that sloshing compound won't work? Also, obviously I, too, am going to need some of that foil tape, so if anyone has a better solution on that, please let me know.
Lastly, I'm going to need a new back-half of my muffler. Anyone got one they want to sell?
Sorry. I know that's a lot of questions.
Thanks in advance, and thanks for everything already answered.