I've HEARD of this, but never was able to substantiate it with anyone that personally experienced it. But, that was years ago...something like 10-12 years ago. And, I'm told that the oil compositions NOW may have remedied that potential problem.

One personal experience doesn't provide credible proof BUT, ..........I changed from 30w dino oil (Havoline) to 10w-30 synthetic (Mobile 1) at 65,000 miles. It was the supposedly notorious 3.1 v-6. I went another 128,000 miles without a problem. This was a vehicle that I quickly put high mileage on due to weekend trips that was approximately 1000 miles overall. The guy that bought it, put another 28,000 on it before it was wrecked in an accident.

 

I can't remember seeing a motor with an oil related failure in a long time (not counting motors that seem to sludge ) . Tranny, is different. I have talked to many people stating after a flush on a high mileage auto, shortly later caused a failure. But not related to using syn.

Biggest issue I have heard on motors is some make leak after switching since syn has smaller molecules.