Community Forums:

There were a few oddball synthetics (including some attempts by major oil manufacturers) that did create a permenant compatibility problem if installed in an engine ( with some even a small residue in the engine would react chemically with other lubricants and turn the whole mess into gelatinous goo or almost water consistincy), a couple of them were pretty decent racing oils. but that was over 20 years ago. I do not believe any of these are still on the market.
I started selling Amsoil around that time and had to work around the bad reputation of other lubes all the time especially among racers who had had the bad chemistry experiences.
You also had some people trying to use synthetic jet engine oil in cars...bad idea ( don't be tempted when you see the stuff in a military surplus lot for $0.15 a Qt.) excellant heat dissipation qualities, but lousy film strength and little if any detergent.

It's OK to switch older engines , just flush it first.
I switched my '90 at about 125,000 , and my '84 diesel at about 224,000 mi.