We have the same car (and my 380SE is also blue). I replaced all the valve stem seals about 2 years ago. Since, last summer, I had a problem with oil fouling the plug on #8 cylinder. Upon going back in and doing the seals on this cylinder again, assuming I had not seated one of them properly before, I noticed that the ridge on the valve guide that holds one of the seals in place was distorted. I pushed both on hard and buttoned it up, with no recurrance of fouling until last month.
Just about a week prior to this second fouling episode, I was going down a hill, stopped at the bottom at the stop sign, and got a huge plume of blue smoke when I accelerated into traffic.
This time, I just changed the plug, figuring that pulling the head to replace the valve guide wasn't worth the trouble. I havn't seen any blue smoke since either. I'll be following this thread to see if anyone contributes a good theory though.

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1986 560SL
2002 Toyota Camry
1993 Lexus