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SolarSlash said:
ive heard stories that "once you go synthetic, you can never go back" to regular oil? is this true? what conseqences are there from switching? thx.

From a lubrication and component cooling standpoint, there's no reason you can't switch from dino to synth and back, or even mix a quart of dino into a sump of synth if you find the level low on the stick. Dino-oil isn't as good an oil as synth, but the two are not incompatible.

In the early days of synths, it was widely theorized that oil seals in the engine were being affected by the synth, having a tendency to shrink slightly due to the slightly different chemical makeup of the base stock the synth was made from. This minor shrinkage tended to pull the seals away from the part to be sealed (crank seal, valve stem seal etc), resulting in a low-grade leak. In fact, I remember back in the 70s stories of adding a quart of automatic transmission fluid to the sump of a car recently switched to synth and subsequently leaking: the AT fluid had/has additives that actually cause rubber seals to swell slightly, closing off the leaks. Fun stuff.

Over the years, synth formulations have changed to provide better seal behavior: to be sure, a bottle of Mobil 1 made in the last two decades is not going to adversely affect the engine seals.

Additionally, dino oil, having a lower-quality base stock, tended to leave varnish and sludges behind as volatiles flashed out. These coatings may have covered internal engine parts, providing, ironically, additional sealing qualities. If a dino-oil engine is then run on synth, the detergents in the synth may actually clean some of the varnish and other deposits away (albeit slowly, and the synth itself doesn't tend to add any deposits of its own), which may lead to "leaks".

So, no, I don't think the old axiom is true. But know that any dino oil isn't going to match the performance of synth oil.
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