I've seen a 95 C36 that had been run on the dino stuff for its first 45K-50K hemorage oil from just about every gasket/seal once it was switched to Mobil 1 15w-50. The car had been driven irregularly and there was a bit of gunk in the engine. Upside of the switch to synthetic was that the gunk in the motor disappeared and now its as clean as a new motor. Of course, the first few oil changes had the Mobil turning dark/dirty after only 3000-3500 miles. The car didn't use any oil before or after the switch. Don't have any info on power or fuel economy changes.
Just my pair of cents...Lee