sharp oil pressure drop-off > 130C

Dec 29, 2010
Paramount, California
One thing that wasn't mentioned much in this thread is the tremendous tendency of viscosity grades like 5W-50 and 10W-60 to permanently shear. Liqui Moly 10W-60 and Castrol 10W-60 spec KV100 = 24.0 cSt and 22.7 cSt, respectively, but UOA's show a much lower KV100. Castrol 10W-60 has about six times the VII content of Mobil 1 V-Twin 20W-50, which means the former will suffer about six times the permanent shear (permanent viscosity loss) of the latter, when the VII polymers are deformed between the fast-moving sliding surfaces. In high-RPM racing conditions in performance engines, this could happen in a few laps. I would guess the permanent shear also happens faster at higher oil temperatures, when the VII molecules are expanded. Likewise Castrol 5W-50 has more than seven times the VII content of Mobil 1 V-Twin 20W-50. So, what you're seeing in the plot in the original post is probably not only a temperature effect but also the result of the permanent shear of the VII as the race goes on. SAE 10W-60 sounds good to the ear, but you need to understand how it is blended and its inherent downsides. Estimated HTFS (base-oil + DDI-pack viscosity at 150 °C) and VII content of selected oils