- Nov 29, 2021
Thank you for bringing this back to J300 discussion.I see your point.
Although XW-20 is more shear stable than say 20W-50, it's HT/FS would be 2.4 (that's pretty thick for the grade) (HT/HS started at 2.9). That leaves higher temperatures, as a second factor further reducing the HT/FS to an alarming figures in extended high RPM operation, which cannot be considered for these high performance engines because it wouldn't bear the elevated temperatures. But 20W-50 even if it is less shear stable, will retain HT/FS 3.5 for instance (HT/HS started at 4.7), and will have more headroom for higher temperatures to continue providing a safe film and minimize boundary lubrication incidents. Add to that, even if the oil permanently sheared, no VIIs, worst case, it's still able to provide a safe, effective film.
On the other hand, HT/HS is relevant to the bearings where VIIs can still contribute to viscosity at this shear rate.
So, XW-20 can be thicker than XW-40 in some cases if we're talking about HT/FS which is relevant to to the valves train and piston rings, but it doesn't mean it will protect the bearings better than XW-40 when it comes to HT/HS.
Both HT/HS and HT/FS have their place in correlation to engine wear and eventually they may be two sides of the same coin.