Synthetic oil - tales of warranty void?

Um ... not really

The SAE J300 spec covers viscosity branding, eg 5W-30, 0W-30, 10W-30 etc

An oil rated a "0W" has the following J300 specs: a maximum MRV pumping viscosity of 60,000 cP - centiPoise - at -40 C, and a maximum cranking viscosity of 3,250 cP at -30 C. Note that one cannot directly compare pumping to cranking viscosity, as they are measured differently

An oil rated "5W" must have the following J300 specs: a max MRV pumping viscosity of 60,000 cP at -35 C, and a max cranking viscosity of 3,500 cP at -25 C

For an oil to have a running viscosity that meets J300 spec "30", it must have the following specs: a viscosity range of 9.3-12.5 cSt - centiStokes - at +100 C, and a minimum high temp high shear viscosity of 2.9 cP at +150 C with 10^6 shears/sec

An oil rated "40" has two running temp specs, depending on whether it is a "light duty" oil, say a 0W-40 or 5W-40, or a "heavy duty" oil, say a 15W-40. The primary difference is HTHS ratings, the LD xW-40 must have a minimum viscosity of 2.9 cP at +150 C, the HD xW-40 must have a minimum viscosity at +150 C of 3.7 cP

The rating of 60,000 cP for cold pumping is the maximum before "yield stress" at that temp, the oil will cease to flow. There are obvious benefits to running an oil with much lower pumping viscosity in low temps. Mobil 1 0W-20 is usually rated around 14,000 cP at -40

Oil quality is a huge can of worms. I'm no longer allowed to attach huge pdf's, surprisingly the API publishes the test data now. The current API rating system for SM is still a joke.

An oil that allows cold sludge, cold stuck rings, a viscosity increase of 100%, is garbage, but it still earns a "pass" from the API. That's your min spec API motor oil, too bad a lot of folks are brainwashed into believing all motor oils are exactly the same

Put the same garbage oils up against ACEA A3/B3, B4 specs. They wouldn't have a prayer. Perhaps that is why in the EU, Toyota has a "normal" 12 month or 10,000 mi oil change interval