I've heard of going conventional to synthetic causing problems with older cars (both on this board and from my dealer), but our cars were converted early in their life, so I don't have any personal experience there.
As one for the interesting stories file, for many years we ran 15W-50 Mobil 1 in our '87 300E, during which time it started consuming oil heavily. The first time, we had a valve job done, and it fixed the problem. It started happening again a few years later, to the point where it was about 1qt every 800 mi. Then, our dealer started using 5W-50 Castrol Syntec, and right at the same time our oil consumption went down to almost zero. It still drips a little bit of oil, but the change on the dipstick is barely noticeable. Very strange..