Do you use synthetic oil? When you clean the flametrap housing, is there burned oil flakes and gunk? I believe that is what clogs the system and makes replacements necessary and causes the main seal leak. I don't think it's a mileage number like a timing belt, and then you have to rip the manifold off (unless you really want to), If you have been using synth oil and don't have horrifying solid flakes of crud in the housing, you might be pretty safe. Also I don't think a little vapor coming from the dipstick is cause for panic. My main seals (twice) were done at 110K and very quiet since. I think it's a matter of good maintenance and even though the previous owner went to the dealer (0 mi. to 110K mi.), it was not enough.
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'97 855na, Mobil 1, K&N, various IPD's and eBay's