Originally Posted By: modularv8
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Come on now. That's a 100% falsehood.
Are you saying the engines running around the last 20yrs and longer with a million plus miles on them running on 10w30 are not durable.
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.
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Then why do they still spec 5w30 and even 5w50 in some of their most modern engines.
Ford has incorporated engine hardware technologies that are optimal for low viscosity lubricants. An OEM specs a viscosity on the design expectations (operating conditions) of their engines. Everyday passenger engines are capable of operating an HTHS as low as 2.2 cP below 120C (248F)at full load (6000rpm/WOT) without any increase in wear rate for all engine components. HTHS 2.6 cP becomes the lower limit when oil sump temperature reaches 130C (266F) and rpm is above 3000. Engines spec'd other than 5w-20 by Ford are expected to operate beyond the operating margin of safety the OEM feels comfortable with. 5w-30 & 5w-50 are spec'd where there is a concern for fuel dilution (EcoBoost) or power output/high rpm is expected (Supercharging, Mustang/SHO).
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Only under certain conditions. the most common oils sold in Germany have a HTHS os 3.5 or higher yet they have no problems with durability. They have had issues in their operating conditions with engines running lighter oils.
Yes, lubricants are optimal under certain conditions. But every study I have read show an increase in main bearing wear increasing HTHS beyond 2.8 cP. I can not provide these studies because of copyrights, but look at figures 2a and 2b of the following link,
Extending SAE J300 to Viscosity Grades below SAE 20
I have these papers, and these graphs were generated at 2500 rpm, 130C, full load in a 1984 GM 3.8L. Main bearing wear increases above and below HTHS 2.2 cP. I have seen in other papers where some engines designed for low viscosity lubricants can go as low as 1.8 cP with no bearing distress/wear. I agree the increase in wear above 2.2 cP is insignificant relative to going below it, but this was in an engine designed more than 30 years ago tested under not normal conditions.
There are reasons that European OEMs have traditionally preferred HTHS 3.5 cP, mainly out of design of their engines. This is changing and there is a move by OEMs in Europe to move in the same direction as the USA toward lower viscosity lubricants. Even motorcycles will be moving toward lower viscosity lubricants.
My previous work at SWRI (where OEM/lubricant industry testing and certificaton happens) and my current research position has provided access to vast industry databases of research, including the SAE database.
Very well said.
I would add, that a main reason the European OEMs tend to spec' HTHSV 3.5cP oils is to offset the higher oil consumption associated with the higher rpms associated with high speed driving. This issue is exacerbated by the long OCIs that tend to be recommended.
