This is the ultimate question asked on BITOG. What protects better -- thin or thick oil? Of course, if you say you should follow the recommendation in your owners' manual, you shouldn't be on this site to begin with. There are many folks here who think that they are better off filling their engine with water than with anything thinner than SAE xW-40. On the other hand, there is also the Dr. Haas school, who believe that thin oil protects better because it flows better. So, why is there still no consensus on this? Which is it? To experiment with this, what is better than a simple but very well-built 1980s engine that was built before even multigrade xW-20 existed. In fact, it doesn't even recommend 5W-30 in warm temperatures. 10W-30, 10W-40, 10W-50, 15W-40, 20W-40, and 20W-50 are the recommended grades, with the 10W-xx grades covering the widest temperature range. I was already getting good results with TGMO (Toyota) 0W-20 SN made by ExxonMobil. There was apparently some antifreeze seep problem and I used ACDelco cooling-system seal tabs before the last oil change, which seems to have alleviated the problem. Oil consumption with TGMO 0W-20 SN was small thanks to new valve-stem oil seals -- about 0.3 quarts in 5,000 miles. The last fill was Mobil 1 (M1) 0W-40 SN (older, non-FS formula). Oil consumption stayed the same or perhaps increased slighly to about 0.4 quarts in 5,000 miles. I was surprised that thicker oil didn't improve the consumption -- in fact made it slightly worse. So, TGMO 0W-20 won over M1 0W-40 in the consumption department or perhaps it was a near tie. Sodium (Na) in fact went down thanks to the seal tabs. However, I still have some coolant loss, probably seeping externally. Wear numbers were the real surprise, even though they were something I long feared and suspected about thicker oil. Iron went from 12 ppm to 18 ppm when I switched from TGMO 0W-20 to M1 0W-40 -- a 50% increase in iron wear when switching from thin to thick. With M1 0W-40, all of a sudden nickel (Ni) appeared, which was near nonexistent with TGMO 0W-20. Aluminum, lead, and copper were similar with either oil. Chromium has improved with M1 0W-40 SN, but I attribute that to ACDelco cooling-system seal tabs rather than the oil, as antifreeze (glycol) is known to cause ring wear. TAN was similar with either oil. M1 0W-40 TBN was expectedly higher, as it has a very high starting TBN. ZDDP (phosphorus [P]) levels were similar in either oil. TGMO 0W-20 had a lot more moly (probably the trinuclear type) than M1 0W-40. Fuel economy was a lot better with TGMO 0W-20 than with M1 0W-40. Engine idled somewhat smoother with TGMO 0W-20 than with M1 0W-40 due to less viscous drag. Conclusion: TGMO 0W-20 SN protects better against wear than M1 0W-40 SN. Iron wear is a lot less. This is probably due to better oil flow of thinner 0W-20, which helps more oil to get to critical parts, such as the valvetrain. TGMO may also have some additive advantages over M1, such as higher moly. In fact other grades of M1, such as M1 0W-20 SN, worry me even more as they have very skinny additive packages. Fancier base stocks (PAO and ester in addition to Group III) in M1 may also have contributed to higher wear in comparison to the Group III TGMO, as esters for example are well known to increase wear by competing for the surfaces with the antiwear additives. Less wear and better fuel economy makes TGMO 0W-20 a clear better choice over M1 0W-40. TGMO 0W-20 may be even better for less oil consumption. However, M1 0W-40's higher TBN might help in extended OCIs (over 10,000 miles). Last but not least, not all engines are the same and there may be some engine that really needs thicker oil. I have also lost my trust in European (ACEA) oils after this. Perhaps that BMW would run better with TGMO 0W-20 SN than with ACEA A3/B4 Mobil 1 0W-40 or German Castrol 0W-40. TGMO once again has reinforced my trust. I think we should give Dr. Haas the credit he deserves. Thin wins.