https://www.bizol.com/company/education/lubricant-knowledge-base/why-synthetic-motor-oil/If the synthetic oil has a viscosity index that is high enough, VI improvers need not be added. In any case, synthetic oils need much less VI improver than mineral oils.
The problem with VI improvers is they tend to shear down with use and lose viscosity at high temperatures. Years and years ago GM had a problem with people using 10W-40 with subsequent engine failure. That was late '70s early '80s. They determined that the VI improvers were coming apart under high stress loads, like in the 350 Diesel, and the oil would lose viscosity and allow metal-to-metal contact. I believe there was even a warning put in owners manuals that if using 10W-40 oil, the warranty was void.
R.
I just read the first bit as I don't have time to read it all right now.
The curious thing is in Germany it's agains the law to palm off as synthetic oil stuff that is NOT synthetic. I've never seen a real Group IV synthetic oil work with alcohol fuels, but the group III oils will. That's because they are not really synthetic oils.
It also stated that Group IV oils (PAO's) are gasoline based. I thought they were natural gas based or alcohol based. Again, I have to check my sources. I may be confusing the Ester based stuff with PAO's.
The reason why it's legal in the US to call an oil synthetic when it isn't can be traced to a lawsuit that went to SCOTUS and a bunch of moron judges who've never lifted a hood let alone looked at a can of oil for purchase decided that refined hydrocarbons are indeed synthetic!!!
That literally makes all oil a synthetic of sorts. The courts should have stayed the hell out of it, but there was money to be made (I should say money to made by the snooker) and in the end, the big money paid enough to grease the skid and make what is not, real.
