Picking a conventional 5w-20: sulfated ash, NOACK?

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Oct 20, 2005
Been using Castrol GTX 5w-20 in my 2011 RX-8. That's discontinued now, so it's time for something new! For the US market, Mazda insists on a conventional 5w-20 that meets API SM with starburst. I know it can take a wider range of products, so I've been researching alternatives. But for now, I still have some time and miles on the engine warranty, so I want to stick with SN/GF-5 conventionals until it runs out. At the same time, deposit control is a big deal for these engines. So, I'd prefer engine oils that are more likely than average to keep things clean. As I understand, sulfated ash is a decent proxy for deposit formation, and NOACK is an okay proxy for base oil quality. Someone please correct me if I'm wrong or missing anything. Either way, I figure cleanliness is a good thing to prioritize because API SN/ILSAC GF-5 will have other aspects covered beyond what Mazda originally specced anyway. I've narrowed my list down to two options: 1. Mobil Super 5000 5w-20 2. Havoline 5w-20 Per PQIA testing (Mobil, Havoline), both have NOACK values under 13%, the lowest I've seen from big brand conventionals that still exist. However, only Havoline lists a sulfated ash number: 0.9% (source), which looks pretty good. I also considered Pennzoil and Castrol GTX Ultraclean. Both make claims about cleanliness that are so strong that I'm tempted to take them into account. I'm just not sure how to weigh those marketing claims against their weaker NOACK numbers and unknown sulfated ash numbers. As of now, I'm leaning toward Havoline just because it's the only big-brand conventional I've seen that shows decent NOACK and sulfated ash numbers -- other than Chevron Supreme, but that seems to be missing Chevron's "Deposit Shield Technology", which seems relevant. Does that make sense? Any other factors or products I should consider?
Originally Posted By: Speak2Mountain
PYB 5w20
Already mentioned. Got anything to add about it?
Another product I considered and rejected: Valvoline 5w-20 lists 0.8% sulfated ash, but PQIA found a NOACK of almost 15%. So I guess I'm assuming NOACK number that's lower by >2 percentage points is better than a sulfated ash number that's lower by 0.1 percentage points. Comments welcome.
Any name brand conventional oil will work fine. All the brands you have mentioned are good. Just keep the oil level full. I'm a valvoline guy, but I hardly ever use the conventional. If I had your RX-8, I would continue with Castrol. The engine will need to be rebuilt after a certain interval no matter how much thought and effort you put into picking the "perfect" oil. You already know Castrol makes a good product, and the new GTX Ultraclean is an improvement to keep up with new automotive technologies and certifications.
Ha, had forgotten about this thread. Ended up going with Mobil Super 5W-20. It's what the shop used when they installed the engine (I think), and it's one of the cheaper options to boot. Guess I hadn't realized this when I first posted, but GTX Ultraclean 5W-20 is a synthetic blend. Doesn't bother me, but it's technically against Mazda's guidelines and I don't want to stray until after the warranty ends.
I've always been a big fan of conventional Pennzoil. I don't know why, but while I'm not brand loyal with synthetics, I'm fiercely loyal to PYB if I want to run a synthetic.
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