Patman
Staff member
How come some oils don't do as well on the HTHS test than you'd think, based on their viscosity at 100c? Here's an example: if you compare two versions of Mobil 1 ESP Formula, the new 0w40 that is factory fill on 2019 Corvettes, and their 5w40, the two oils both have a viscosity at 100c that is not that far off (12.9 vs 13.4) but yet the HTHS of the 5w40 is significantly higher, at 3.8, compared to 3.53 for the 0w40. And by the same token, the 5w30 ESP Formula I'm using has a slightly higher HTHS of 3.58, even though it's viscosity at 100c is lower than the 0w40 (12.1 vs 12.9) What is causing the 0w40 to slightly underperform in the HTHS test compared to the other choices? Is it a different base stock? Less VII?