Let’s see if I can do this without offending you.
1. There is never, ever a reason to run a single grade oil. Ever. The Viscosity Index Improvers since the late 1970’s have been improved. So has the base oil.
Any engine oil that is multi-grade (there is no such thing as oil “weight”) starts out its life as the lower grade. A 20w50 starts out as a 20 grade oil and VI improver are added (among other things) so that the oil acts like it’s higher grade at 212 degrees.
Thats why you never saw a 0 grade oil until not that long ago. The VI improves and base oils weren’t good enough to start with that grade of oil.
At one time Castrol made a 10w50. I used to use it. They stopped making it around 1984ish (IIRC) because the bad oil wasn’t capable enough and the VI improvers had issues with that range.
Today, with a QUALITY synthetic oil (that means Group IV and V base oils ONLY) and todays VI improvers you can get a 5w50 and it will outperform ANY single grade or 20w50 in every single test there is.
You can also run a 0 grade base oil (I run either a 5w30 or probably will run a 0w20 this time) which means you can close up the clearances, carry the same oil pressure and get better cooling and protection than you can with a heavier grade oil. You couldn’t do that in the 1970’s. Or the 1980’s.
The “synthetic“ oils Chrysler was testing are not the same oils we can buy today. So that testing is obsolete. So is the advice that single grade oils offer more protection than multi grade oils. They don’t.
The government didn’t “ban” zincdiakyldithiophosphate (ZDDP). As I said above, ALL ENGINE OILS HAVE ZDDP OR THEY ARENT ENGINE OILS. The government mandated lower levels of ZDDP because no matter how good with a bone you are, no matter how good your rings are, there is oil migration out the exhaust. The ZDDP coats the catalyst and kills it.
ZDDP isn‘t just for cams. It’s for the rings, the exhaust valve guide and any other extremely hot, high load parts.
So there is no “BAN” on ZDDP.
There are many middle of the road, will work well enough oils out there. There are very few high quality oils out there. And you don’t find them at box stores and crap holes like Walmart.