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.
