M1 EP 0W-20 is priced $3.74 after $2 rebate. PUP 0W-20 is priced $10.03 after $2 rebate. This is a huge price difference to justify PUP.
Recent TDS's showed that PUP 0W-40 had a NOACK of 13% and PUP 5W-30 11.5%. These rather unimpressive NOACK numbers show that its GTL base stocks are of rather low quality. They are more Group-III-like than GTL-like. On the other hand, M1's Group III+/IV/ester base stocks have a NOACK of 9% or less. In particular, M1 EP is half or more Group IV; so, its NOACK is probably around 7%. GTL is a form of Group III -- call it Group III++. However, Shell's Qatar/Pearl GTL base stocks probably don't even qualify for Group III+. They could be more like regular Group III. GTL in principle could be of high quality, close to Group IV (never equal though), but this is not necessarily so. Once again, Shell's Qatar/Pearl GTL base stocks don't seem to be of high quality. They probably couldn't figure out how the run their reactors efficiently and/or don't have the patience/economic justification to produce higher-quality GTL.
M1 EP is also guaranteed for longer OCIs thanks to its Group IV and strong additive package.
Infineum additive company is jointly owned by ExxonMobil and Shell; so, there is usually a tie in the additives. However, M1's additive package is hard to beat.
I wouldn't put too much trust in the initial GTL base stocks, which are of mixed quality, in comparison to M1's proven and well-tested technology.
Castrol? Their products vary in quality so much that I don't consider them. There are just too many dismal Castrol UOAs out there. The only think Castrol I trust and use is the DOT 4 GTLMA brake fluid, which is only found in ARCO/BP gas stations nowadays.