Quockieo:
"But..1) if one is buying a used car that has seen many miles, i would not switch.
2) After 100,000 miles, tolerances, clearances, bearings, seals,etc. are all worn/increased in size due to time and usage. You run a high risk of a potential leakage due to the slippery properties of synth. As a result of the potential damage of a leakage, you'll probably have to replace rear main seals, transmissions, or even a new engine which are very expensive to fix."
I've switched engines to synth after as many as 190,000 miles. Have switched many engines to synth at over 100k. I've switched back and forth with impunity, in fact. It's never caused a problem for me. I think the concerns you bring up are not really issues unless an engine hasn't been maintained well in the first place - in which case perhaps a switch to synth can cause problems that were masked before the change: but that could happen at only 50k miles, too.
Personally I don't think synthetics are worth it for most people and most vehicles though, EVEN IF you want your "baby" to last forever and cost isn't an object. Some of the exceptions have been mentioned here. Those notwithstanding, it seems to me that ordinary dino oils have become good enough to match synthetics in all longevity-related areas of performance.
- Glenn