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?