Originally Posted By: SubLGT
Because there is a correlation between MRV viscosities and oil pressurization time at below freezing temperatures.
http://papers.sae.org/912337
I think you and I are reading that data differently.
That study is primarily concerned with the behavior of oils at borderline pumping temperatures and whether the "W" grade limiting temperatures set forth in SAE J300 (The CCS test temps) appropriately reflect borderline pumping temperatures for HD diesel engines. Since the MRV tests for J300 were developed using light duty gasoline engines there is some question whether those paramaters apply to HD diesels running used oil, thus the reason for so many used oil samples in the study.
The vast majority of data in the study is taken at or below the "W" grade limiting temperature set forth in J300. The authors state "Low ambient testing was geneally performed on oils at the "W" grade limiting temperature and 5*C lower (e.g. -15/-20*c for SAE 15w40's)" This is the data presented in the majority of graphs in the body of the paper.
What we are discussing here is using a 0w oil at 5w limiting temperatures of around -25*C, we are not really flirting with the pumpability limits of a 0w oil. With that in mind the only data from that study which seems relevant are Figures 23 and 24 just before the conclusion. Those figures show that as long as oils are used above their J300 limiting temperatures, pressurization time is more dependent on kinematic viscosity than MRV viscosity, which supports what I said earlier.
Look at figs 23 & 24, note how at -20*C the pressurization time for the 10w30 lies right on the trend line established by the 0w30. This despite the 10w30's MRV being six times that of the 0w30 (6 Pa-s vs 1 Pa-s). It's only when you use 10w30 at -25*C that pressurization time increases dramatically. This rapid increase in pumping time is the limiting behavior refered to by the authors. This is true of the 40wt oils as well, when used above the J300 limiting temperature all the 40wt oils lie on the same trendline.
The authors specifically note that the 5w40 synthetic does not exhibit this limiting behavior owing to the fact that its MRV is consistent with a 0w oil. This is exactly as I stated above when I said that any good 5w30 synthetic would work fine in the OP's application. Most synthetic 5w30 oils exceed the 5w MRV requirements by a wide margin, It's the CCS value which prevents them from being 0w30s.
Modern port-injected gasoline engines are enormously more capable of starting in the cold than the carburated engines the CCS requirement was developed to represent. The Toyota in question was presumably designed to start at -10*F (-23*C) on 5w30 conventional oil that barely meets the minimum requirements of J300. It will be slightly harder to start on synthetic 5w30 than 0w30, but it would still have no trouble starting, assuming the starter and battery are in good condition. It will also be safe to do so due to the good MRV performance of 5w30 synthetics.
Back to the RL vs Mobil 1 0w30 question. RL is thinner at all temps below 100*C and above the temp at which the pour point modifiers dominate the viscosity index, meaning at temperatures between say -10C and 100C. This means the pressure relief valve will close sooner with RL than with Mobil 1, providing full oil flow to the functional components sooner. This is the reason RL would be my choice for this application. The MRV viscosity of both oils is sufficiently below the limit to be of little consequence. If the OP were starting the vehicle at -30*C regularly I'd change my tune.
The lower HTHS of Mobil 1 will provide a slight bump in FE over RL, but as others have said you could mix in some 0w20 and have even better performance at intermediate temperatures while having the same HTHS as Mobil 1.
All that said, we really are splitting hairs between these two oils, your truck will live a long life on either.
