Engine Oil Discussion

Charrlie_S said

The factory recommended oil is 10-30 non synthetic., and that is what I use in all my slants. The only thing wrong with synthetic, is it will have a tendency to leak more, and is more expensive. I wouldn't worry about using added zinc, unless you have a fresh rebuilt engine. I would not use a straight grade oil, and definitely not SAE 50. If your engine needs SAE 50, it is time for a rebuild.

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The "problem" with synthetics is also because it will damage seals/gaskets, right? I've heard synthetics will clean the engine better than conventional oil which causes the engine leaking, because of "missing impurities", which "sealed" the engine, but I don't know what's true about that.
The brand, Motorex (a Swiss brand), produces special oil for old cars. One of those oils has extra zinc, as I've read worn out/not rebuilt engines actually require zinc, so I was thinking about that kind of oil. It is also available in 10W30 or 15W30.

So, a non synthetic oil is "better", but my user manual said i shouldn't add straight mineral oil. This part still confuses me a bit...