If I didn't have any VOAs on hand I would use the Castrol. This is because I've used Castrol before and there are just too many oils out there to try them all long enough to know if they will prolong the life of an engine. One would have to assume that an M rated oil is better too.
Is this a trick question or something?
The API SM Castrol GTX would be the absolute hands-down winner, even more so because it's cheaper than the older Pennzoil.
I'm not even sure why you'd consider paying more for the Pennzoil with the SL rating.
Do we have any VOA's of the Castrol 5w-30 SM dino?? If not, purchase that one and paste the results in the VOA section.
Nothing against PZ, but if it's SL, it's not worth it at this point due to the overwhelming amount of shelf-clearances at most places to get rid of old stock. If the price drops substantially, then I'd buy it.
From what I can see SM oil are kind of new and unproven at this time. I'm not to impressed at what I have seen on the SM VOA so far concerning SM additive package. SM oils should be cheaper because they are spending less on additives from what I can see. But very little information so far. Never did like being the 1st one with a new product.