"Which endurance races, and what engines, bearing and piston clearances, piston type, valve train design, oil temperatures, fuel type, any dirt ingestion issues, and do they want to tolerate extreme fuel dilution issues and keep on racing? And that's just for starters. "
Most people start with the basic tenant that thicker oils tend to produce thicker oil films, and thus less wear, and then reduce viscosity short of breaking something or excessive wear. Thta's why we see very thin oils being used for qualifying, thicker oils used in 'sprint' races, and even thicker ones in endurance races. One uses a thick enough oil to provide needed protection, regardless of whether the problem is high heat, high wear, fuel dilution, and such. Thta's why thicker oils are still used where maximum engine life is needed, and that's why thinner oils are used where improved fuel is more important than maximum engine life. Referencing Taylor again, we see that across the board thicker oils tend to produce thicker films, at the primary wear points investigated.
Only in 5w20 bizarro land do people use thinner oils in a race than when qualifying, and thinner oils in an endurance race than in a 'sprint'.
http://iantaylor.org.uk/papers/SAE2002013355.pdf
Lubrication, Tribology & Motorsport
....For most conventional European gasoline engines, the maximum rpm is around 7500 rpm, typical mid-range displacements are around 2.0 litres, with a bore-stroke ratio close to 1. The most common engine configuration would be an in-line 4 cylinder engine, often with 4 valves per cylinder.
In comparison, a Formula 1 engine has a displacement of 3 litres, with a V10 engine configuration, a bore-stroke ratio in the range 2.0-2.5, and a maximum rpm of 17000-18000 (these latter figures were reported by Wright15).
....Clearly, Table 6 shows that reductions in piston ring friction can be achieved by moving to a lower viscosity lubricant. However, this is at the expense of lower minimum oil film thicknesses. There is a trade-off between reduced friction (and greater power available to the wheels) and engine durability. In Table 6, we can see that moving from an SAE-15W/40 lubricant to an SAE 0W/20 lubricant will lead to a 30% decrease in top ring minimum oil film thickness, and will also give a decrease in top ring friction of approximately 30%. The benefits of moving to lower viscosity lubricants to obtain better fuel efficiency via lower engine friction have been well documented24-29.
Lubricant, Minimum OFT uM (Oil Film Thickness) , Average Friction Power Loss (W)
SAE-20W/50 6.89 671.5
SAE-15W/40 6.27 577.7
SAE-10W/30 5.15 425.1
SAE-0W/20 4.55 348.4
Table 8: Sensitivity of con-rod bearing results to lubricant viscosity grade at 7500 rpm
It is recognized that the model used here is too simplistic for use as a detailed design tool of bearings.
.....Lubrication in the valve train differs from that in the bearings and the piston assembly, since it is generally accepted that the valve train is in the mixed or boundary regime for most of its operating conditions. This is certainly true as far as conventional engines are concerned. Measurements of valve train friction torque frequently show that, under normal operating conditions, the valve train friction torque increases as the viscosity of the oil decreases. It is also in the valve train that friction modifier additives are thought to have the largest effect.....The boundary friction torque may be calculated by estimating the oil film thickness using elastohydrodynamic theory, and then comparing this value to the combined surface roughness of cam and follower, and assigning an effective friction coefficient. This approach has been used successfully by researchers from Shell39, Ford40 and Nissan41, and is found to give good agreement with experimental measurements... A lower viscosity lubricant will give a higher boundary friction torque, whereas the hydrodynamic friction torque will be lower. Friction modifier additives can be added to the lubricant, and these are effective at reducing the boundary friction torque.
