1. The temperature for an HTHS measurement is done at 150C or 302 F, which is the average peak temperature likely to be encountered in a bearing.
2. The oil is mechanically sheared at a rate of 1 x10^6 shearing operations/second.
3. Minimum Oil Film Thickness measurements (MOFT) of operating engines did not correleate well with actual wear in service. A method was devised by which the oil temperature would be elevated to worst case and sheared to determine the optimum viscosity which better correlated with wear.
Three Exxon Researchers found that a minimum HTHS of about 2.8 mPA.s was the MINIMUM HTHS viscosity needed for normal wear, with the higher the HTHS being better for minimum wear. IN general, the higher the viscosity, the greater the HTHS.
For example, in a fleet of taxicabs using a GM 4.3L V6 engine, if the HTHS was 2.35, the startup film was 0.097um and 2.56 um at running; if the HTHS was 2.98, cP, the Startup oil film thickness was 1.231 um while the running film thickness was 3.22 um.
In Dynomometer wear tests using four GM 3.8L engines, the wear mass of a connecting rod bearing was as follows:
mass loss (gm.) - 190
mass loss (gm.) - 28
For "mains" bearings:
mass loss (gm.) - 150
mass loss (gm.) - 40
A jump in HTHS by about +1.5 results in approximately 1/5 the wear. Now this relationship is not linear and flattens as one nears a 40+ weight oil.
I should also mention that this test showed little differences in wear between a high quality 5W20 and a 10W30 for oils of close HTHS. For example, The average wear of one of the 3.8L V6's showed a total wear of the Connecting Rod bearings as 48.4 grams for the 5W20 verses 44.3 grams for the 10W30. For a 10W40 oil, the wear was 39 grams!!!
[Mola's comment: I think this test verifies my earlier comments that most daily driver engines can use any oil from a high quality 5W20 to a 15W40 fleet oil.]
Summary: It was found that HTHS correlates better with wear values found in actual oil analysis and actual tear-down measurements than does measuring the oil film thickness in situ.
I too think that viscosity at any given temp. while a factor is not the most important! I think that when compareing wear HTHS and MOFT are the most important things we consider wear rates with different viscositys. Redlines 5W20 has an HTHS of 3.3 while M1 0W30 has an HTHS of either 2.9 or 3.0. SO I would expect the Redline 5W20 to perform better so long as presure adn flow were within spec. If a vechile's oilpump can not maintain adequate flow and presure then the thin oil even with the higher HTHS is not going to do you any good!
I belive that TooSlick's poor results with 5W20 were do to flow and presure issues in a system not designed for 5W20. DC had to re-clearance their G-Rotor oil pump on the 4.7 and 3.7 from it's origanal spec. whena at the last minute they decided to make 5W30 the prefered grade instead of 10W30. The rest of the engine remained the same.