[Rich Z]

Find myself reading about oil again, and found an interesting quote:

No. What I said was that engines designed and built for low viscosity lubricants have reinforced features and better technology that do extend the life of an engine. For instance, unlike the trimetal bearings of the past, low friction engines use AlPbSi bearings. These are a "harder" material that minimizes "flexing" and distribute bearing load/peak oil film pressure much more evenly. The alloy is also much more resistant to bearing seizure. The durability of this type of alloy is regarded by Mahle, Clevite, and others in the bearing industry to be long-lasting and capable of achieving 300k with minimal wear. Low friction engines also employ low-tension piston rings that minimize friction compared to older designs. Low viscosity lubricants work well with low-tension rings to bias the lubrication regime more toward hydrodynamic verses mixed/boundary. Film thickness is the primary factor for wear at the piston ring pack. Low viscosity lubricants provide a greater film thickness vs higher viscosity lubricants. As a consequence of the higher flow characteristics of lower viscosity lubricants there is lower cold-startup & operating wear because of lower ring tension.

Engines designed and built for lower viscosity oils are more durable than yesterday's engines because they are just better made, tighter manufacturing tolerances, better metallurgy, better bearings, etc. Is this technology and new hardware applicable to engines spec'd with higher viscosity oils? Yes. Will running a heavier weight oil in an engine designed for a 20wt lead to lower wear? No. Could it increase wear? Yes. Why? Because OEMs design their engines for oil flow characteristics for an operating range of 80C to 100C. This is where it is expected an engine will operate for the majority of its life. It is also the temperature range where lubricants provide their best protection and efficiency. So now you have the steady-state wear rate that is dependent mainly on film thickness (steady state is mainly hydrodynamic). When we talk about wear and engine durability, the primary component suject to the most wear and arguably the limiting factor for durability is the piston ring pack. Engineers design these for a flow rate to the ring lands to attain a target film thickness based on engine rpm (piston velocity/shear) and steady-state operating temperature. If piston/cylinder lubrication is "flooded" then there is increasee oil consumption. If it is "starved", then mix/boundary condition (increased wear rate). All oil channels are metered for an expected volume. Going too high a viscosity of oil in an engine designed for 20 wt will shorten engine life. How much more wear, I don't know. One thing is absolutely certain and well known by industry experts, nothing increases the wear rate of an engine (under normal operating conditions) than operating above 2500 rpm.

Every SAE published industry study on the subject of engine wear rates involve studying the effect of load and engine rpm on wear rate. In every instance, rpm below 2000 and full load (WOT) and oil sump temperatures as high as 130C did not change the wear rate. Above 2500 rpm for any viscosity oil, at ideal operating sump temperature, always increased the wear rate. It is increased anywhere from 2x up to 10x depending on rpm. This is because of increased shear rate. Main and rod bearings are far more tolerant than piston rings. Piston velocity, piston/cylinder temperature, and combustion chamber pressures increase dramatically when essentially racing an engine. Fortunately, everyday engines don't live at high rpms. Every engine has a finite life.

Source: http://www.bobistheoilguy.com/forums...pics/2710312/2