Originally Posted By: Brons2
Thinner oils work great in engines that were designed for them.
And, the physics of your assertion are wrong. A bearing for example only needs a thick enough oil to form a hydrodynamic film. More thickness beyond that will not do anything other than create more heat.
The particles that are held in suspension are much smaller than these same clearances.
Don't believe me? Check the used oil analysis section for people towing big loads with Ford and Chrysler V8s using 5w20s, or with Toyota V8s using 0w20.
Using a typical late model that specs 20wt, I would say that the entire package is "designed" for thinner oils - not necessarily the engine. As I have stated in previous posts, the cooling, fuel, and overall design allow the use of 20wt oils. In your truck example, if the truck towing a load is able to maintain a 180-200 *f oil temperature, he's running at a 30 wt to high 20wt oil.
Also, regarding your correction, as the path of the crank is not uniform, under heavy load conditions, the space between the journal and bearing can most certainly be scored by dirt! Under extreme use, the oil film can be compromised entirely, causing metal to metal contact (and any particles in between!). Here, the additive package takes over to protect the metal surfaces.
Bruce did an outstanding job creating a 0W10 that seemed to work decently in Gary's 2.5L Jeep engine. His used oil analysis weren't too bad running this oil, but remember - used oil analysis are a tool to determine OIL condition. We can get a decent snap shot observing typical wear metals to roughly determine engine condition, but it is a rough observation at best (IMO). Certain engine designs have historical wear patterns (ex. the Jeep 2.5L/4.0L Fe condition), and we compare our used oil analysis to these figures. We throw around assertions that our engines are in great shape due to nice used oil analysis (and I'm just as guilty!), but the fact remains that while a used oil analysis can look good, there can still be engine damage present. A great example is by a member called BuickGN, who had a nice used oil analysis with half a piston skirt in his oil pan.
Remember, we've been using 20wt oils for decades and decades. I believe we may eventually see common 5 and 10 wt use, as soon as the manufacturers are able to balance form and function to maximize efficiency, in a cost effective package. Emphasis on cost effective!
