Re: Group III vs Group IV (PAO) Basestocks in the
TooSlick: Thanks for the comments. I think that you are right on the mark with regard to the VI modifiers and the breakdown of PAO vs. III. I suspect that the III has heteratoms plus probably has residual unsaturation due to benzene ring type compounds (that hydrotreat may not whack) and both of those can eventually lead to varnish. TS when you say PAO basestock (re: B4) in your last post you mean PAO/ester, right? I find it very interesting that an addition of a IV base may lower ring deposits.
SoTxBill: Good comments, thanks. I think that your argument about additive solubility is fine for PAO but I am not sure if it holds for PAO/ester. Same point in the end though. I would tend to think that the ester addition would create a new problem of additives being too soluble and not coming out at the right time, although I don't know for sure this is just my speculation. It seems that the motivation for blending III/IV could be along these lines, i.e. to make the additives "act right" solubility-wise. I think that the blends you mention are OK as long as there is no VI modifier.
I am changing my thinking about oil entirely. There is no right general answer to the question what oil is best. The answer depends on the situation. As an illustration suppose that large diesels have lower piston ring temp. due to their sheer size (ring placement, ability to get rid of heat?). I would say that a III, IV, or a blend of the 2 would be OK. From a wear standpoint, with no cold start worries, maybe the best order of preference would be III (including blends), IV as Bill suggests. Our TDIs are relatively small engines and might (I don't know) tend to have higher ring temps. or other characteristic (higher oxygen radical concentration from higher boost, less EGR, whatever) that enhances ring deposits. The scale then tips this way in order of preference: IV, III (III including blends).
I don't think we know if III, IV, or a III/IV blend will be best at this point. Some of us will be leary of any III in a TDI due to ring sticking.
Or is it pick your poison? I'll make up a number for the year 2033, location South Florida: 937,000 miles on my TDI running GIII and it goes due to piston ring sticking. The guy down the street gets 937,000 running IV, rings don't stick, but failure due to sheer wear. The girl next door has some of both problems running III/IV blend at 937,000.
I think that we are at a standstill here until we have some data to settle the III/IV issue. Until then......?