Engine Oil Discussion

SlantedMark4 said
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...

Synthetics will not damage seals and gaskets. Early synthetics had two oil base stocks. One is used in AC systems as it works well in the low temperatures. The other works better in hot conditions. Some of the early manufacturers used only one which led to problems.
Amsoil was the first to market with an automotive synthetic oil. Amsoil blends their base stocks to get the desired properties, and then uses a premium additive package.
Now the zinc issue. This is not a matter of high mileage or wworn out engines. It is the flat tappet cams, whether hydraulic or solid. The small high load contact area requires the ZDDP of 1.500PPM to about 2.000PPM to prevent wearing the lobes off the cam.
I have my Ford 289 apart and the cam is showing signs of the cam just at the initial stages of failure due to the lack of ZDDP in the oil. This engine has not been run in 10 years or so. The cause of cam failures was not known for a time to be the removal of the ZDDP. This happened due to catalytic converter incoropration for emissions. A newly rebuilt engine was extremely suseptible to cam wear as the parts wore in and decided to like eachother. The issue was not as evident on older engines that were worn in and spring pressures were not high as a performance built engine would have.
The Shell Rotella T4 oils should be available in Switzerland off the shelf, and be a lower cost option for you. It is a 15W 40 oil, but that should work fine in your slant.
Newer engines use roller followers that work with oils without the ZDDP. Many overhead cam engines use bucket followers with enough load surface to tollerate the lack of the ZDDP. I believe metallurgy plays a big part also. NASCAR when limited to flat tappet cams used DLC coatings to get them to last under the high spring and RPM loads. Expensive process.