The other night I noticed that Widman has updated his Oil for Corvairs paper and now is a little more generally about flat tappets and includes more on High Temperature, High Shear viscosity (HTHS). Limited comments about specific oils but Dr Charles should look at page 24.
Very interesting read, thank you. Also the top of page 29 Back to VR1 10W-30!
And you can mix 20W-50 in the same if it looks like that's needed. it would get something like a 15W-40 so not such a big experiment along with its higher HTHS
