Originally Posted by dnewton3
The topic of shearing is an anecdotal input.
What we should be caring about is the wear metals; the output.
All the data shows that wear rates are essentially identical between 30 and 40 grade lubes, holding other variables constrained such as base stocks, environmental use, etc.
Vis is like FP and other inputs; it's only a predictor to warn of potential shift in wear rates.
Wear rates, themselves, are what tell you how well things are going in the engine.
For most any OCI the typical BITOGer is likely to practice in his 6.0L PSD, the topic of Vis is moot.
I totally agree w/ the above. The whole shearing issue has been overblown. That said, the discussions will no doubt continue. Shearing rates will continue to be exaggerated on 5W40 oils, and 10W30 oils will continue to be mistakenly assumed to be miraculously immune.
That said, one UOA I recently saw showed wear metals (copper, iron, and perhaps lead) just starting to rise (based on UOA history), but the absolute numbers were still not bad. IMO they were showing the early signs of atypical wear for that engine. The oil was a 10W30 and the viscosity was at 9.6 cSt (almost a 20wt). His mileage on the oil was 16k. He has chosen to keep running it.
No doubt he is one of the "fringe" with 16k + OCI, but I wouldn't say that it was excessively out of operating ranges that many people chose to go (I see a number of people at 10k OCI). He chose this course based on the assumption that 10W30 doesn't shear and that, with his engine, he is focused on the absolute numbers (trends aren't what he will use for decision making).
The fact is, when you are operating on the edge, there are abnormal and unanticipated things that can happen that could become costly when perhaps they wouldn't have been if a conservative approach were being taken. For example, a coolant leak can unexpectedly send temperatures upwards while driving on a long trip. Unexpected fuel dilution from an injector is another possible problem. Operating on the edge of your oils capability, because normal conditions say it is acceptable, might be OK in industry where things are done statistically and the entire maintenance program is designed to protect against many failure modes. That is simply not true for the average vehicle owner. Most individuals would get hit hard from a catastrophic failure of a 20k (+) vehicle. With the 6.0L, caution seems to be even more warranted, lol.
Personally, I appreciate your response above. As you know, statements like "the 6.0L prefers a 30 weight" tend to get me started.