"This paper is 27 years old. Viscosity has been shown (today) to have nothing to do with wear in fully formulated oils."
Whether three, two or one decade old, or even a year old, one finds that viscosity is typically a major factor regarding wear. That's the way oil and bearings work, otherwise we'd see applications begging for better fuel economy, like over the road diesels, using 5w20 instead of the 15w40 that they've used for quite awhile. We'd also see 5w20 having been used in places where fuel has been expensive for a long time, like in Europe, but instead heavier oils have been the norm. In racing one would see nothing but 5w oils being used for better performance, but instead one sees examples like Shell using a heavier oil in F1 Ferraris for practice in order to get the engines to last longer. 5w20 will work fine for almost everyone in the US, speeds aren't that high and drain intervals aren't that long.
http://www.sae.org/technical/papers/922342
Engine Oil Viscosity and Bearing Wear: Field Test Results
Document Number: 922342
Date Published: October 1992
Abstract:
Two programs were conducted to study the relationships between engine oil rheology and crankshaft bearing wear. A chassis dynamometer test of four oils in four cars was used to explore and define the key variables affecting bearing wear. There results were used to design a field test of nine oils in 45 taxicabs in New York City. The test oils (SAE 0W-20 to 20W-20) were formulated to measure the effects of viscosity, viscosity index improver, and detergent inhibitor package. Bearing wear tended to be either low and unremarkable or very high, particularly in the thrust bearings. Oil performance was best expressed as the frequency of excessive wear, rather than by quantitative wear measurement. There were many instances of very high wear in cabs operated with the lowest viscosity oils but none in cabs with higher viscosity oils. Non-Newtonian oils appeared to provide slightly more protection than Newtonian oils of the same HTHS viscosity, and a higher quality adpack also appeared to provide benefits. However, these factors were secondary to the viscosity of the oil. HTHS viscosity was a better predictor of beairng wear performance than oil film thickness.
https://shop.sae.org/technical/papers/980702
Properties of Engine Bearings Lubricated With Low Hths Viscosity Oil
Document Number: 980702
Date Published: February 1998
Abstract:
Properties of engine bearings were investigated with different bearing materials and different HTHS viscosity oils by means of both an engine test and a rig test.
The rig test well simulated the bearing wear which occurred in the engine test. Lead-bronze bearings with lead-tin-indium overlay gave the least amount of wear in operating under high-speed and heavy-load conditions even with low HTHS viscosi Aluminum bearings without overlay gave good wear resistance in the case of no seizure occurrence. The wear amount of bearings were well correlated with HTHS viscosity, not with kinematic viscosity.
SAE Technical Paper Series. Design Considerations in Formulating Gasoline Engine Lubricants for Improving Engine Fuel Economy and Wear Resistance Part 1 Base Oils and Additives. 2007-01-4143
19. Adequate performance for a lubricant in the VIB sequence is most important to most OEMs as this is a good indicator of fuel economy, after which sequences to evaluate the wear performance are then considered.
20. These new lubricants must balance the fuel economy improvement FEI% and good wear resistance.
37. But base oil does have an effect on hydrodynamic friction which viscosity and film thickness which is good for wear, bad for FEI.
http://www.joegibbsracingoil.com/trainingcenter/082106.html
The oil wedge formed in a hydrodynamic bearing is a function of speed (RPM), load (cylinder pressure), and oil viscosity (at operating temperature). Under fluid film conditions, an increase in viscosity or speed increases the oil film thickness and the coefficient of friction, while an increase in load decreases them. The separate consideration of these effects presents a complex picture that is simplified by combining viscosity Z, speed N, and unit load P, into a single dimensionless factor called the ZN/P factor.
