[darrin]

The OP raises an issue that's outside of the normal "what's the best oil" post and that I don't recall seeing addressed before (though I've not read more than a few pages of the "best oil" threads) -- would introducing synthetic into a car with older oil lines/seals/etc. that had only "seen" conventional dyno oil encourage the development of oil leaks, etc.

When I originally got my car in the 90s, I recall being told that conventional oil caused parts of the lubrication system to expand and that switching to synthetic could cause these parts to contract and leak oil. This caused me to stick with dyno until I had my engine rebuild, at which point I switched to synthetic.

Is there any disadvantage to using synthetic oil in a car that always used dyno (or, conversely dyno in a car that always used syn)?