Originally Posted By: d00df00d
Makes sense! Thanks, SonofJoe.
More not-even-half-informed ramblings to follow:
One of the practical questions in all of this is whether I'm barking up the wrong tree by pursuing high-end low-ash synthetic oils. My thought had been that an oil meeting a spec like MB 229.51 or 229.52 would offer the best chance at keeping the engine clean while retaining performance at-or-above the SN/GF-5 level in all other respects (except mpg).
If I'm understanding better now, using a low ash oil would mean the oil itself would contribute minimally to ash formation. Such an oil might also contribute minimally to other kinds of deposit formation IF the finished oil were sufficiently oxidatively stable despite the light additive package (e.g. thanks to highly stable base stocks). Is that correct?
If so, I think I understand where the fellow at Amsoil was coming from. He kept stressing that the vast majority of the deposits will be coming from the fuel. Might a lower-ash oil be less well equipped to control those deposits, all else equal? If so, might I be able to compensate by just keeping my OCI short?
Tangential: I've also read that low-ash oils can reduce intake valve deposits in DI engines quite significantly. Is that mechanism sufficiently unique to DI engines that it's meaningless here? If not, how does it fit?
Really appreciate you guys wading through all this and helping me understand!
Okay, first off, you're looking at deposits in a Wankel engine. I, and probably every single senior oil formulator in the entire world, knows precisely naff all about deposit formation in rotary engines because they're so rare & no sane person would every expend the kind of money you need to properly understand them. These engines are very likely a law unto themselves and I'd guess that the only people that have any kind of fix on what impacts on deposits are Mazda themselves.
Taking the more general points you made in no specific order...
Regarding the 'deposits come from fuel' thing. What can I say? It's lifted straight out of the Big Bumper Book Of Business Excuses! When in doubt, blame something you're not responsible for. Does gasoline quality impact on engine deposits? Quite possibly yes but that's not why the excuse is being deployed which is to portray the oil as non-complicit.
Regarding low ash oils and Inlet Valve Deposits, there's an old Lubrizol presentation written by a guy called Mike McCabe that infers that low SAP oils reduce IVDs. Our own Brother Gokhan, in his uncritical naivety, posted this presentation up a few months ago like he'd discovered something of massive importance! Sadly what BG failed to pick up on is the author works in Marketing and the presentation was a 'marketing' presentation in support of Low SAPs oil as a new, and potentially profitable, market segment. The lubes industry absolutely loves market segmentation! It's a way of sucking more money out of the poor saps that buy engine oil. Personally I would totally disregard the 'low SAP oil means less IVDs' conclusion this presentation draws. It's standard practice in these matters to organise test work such that it draws you to a preordained, sleight-of-hand conclusion. Nuff said...
Regarding MB229.51/52 oils, it's been several years since I looked at these in detail but these will be top flight oils made from excellent base oils and loaded up with additives. As such, they will be better than your average GF-5 oil. You won't hurt your RX-8 by using them.
Finally, it maybe stating the obvious but low ash oils DO indeed produce less ash and have a reduced tendency to plug Diesel Particulate Filters (DPFs). I would add my normal health warning here which is that low Ash does not mean NO ash (or even that much lower ash than 'normal' oils!) and that even with low SAPs oil you can typically expect a DPF to be knackered after 80,000 miles. However it's wrong to conflate DPF plugging ash with engine deposits. I'll say why in my next post...