This link would be an example of Big oil(pennzoil ultra) admitting in their sales pitch that motor oil in the past/present was the culprit of sludge and not the engine designs during all those years of massive sludge issues.
Why wasn't big oil ever sued during those massive sludge years? Well,apparently the truth wasn't known during that time,and now it is finally revealed. Too bad the statute of limitations has run out,otherwise they(big oil) would have been sued at the start.
Pennzoil is comparing two standards(old/present vs new) on the same engine block with the old motor oil standard showing heavy sludge on the block face in the link below, proving that the old standard, was causing sludge all along,thanks to their present sales pitch revealing this info regarding sludge.
This newer standard is with only group III synthetic proving that the superior group IV 100% synthetic was better all along,and anyone who used,and is presently using a group IV is making the superior choice.
If a group III is good(P.Ultra)......then a true group IV has to be....GREAT, now and then.
Here is what BITOG Thinks regarding dexos vs P.U. Remember,even the dexos standard is still not really a 100% PAO oil,making group IV the smartest,safest bet.