[Ben99GT]

High Temperature High Shear viscosity, learn it, use it. It's a viscosity measurement when exposed to heat (302 F) AND shear/pressure, as in the main and rod bearings. The SAE/kinematic viscosity measurements don't take shear into account.

There are 5W-20s out there that have higher HTHS viscosity than just about any of the off the shelf synthetic 10W-30s.

That's why I like Red Line 5W-20 for Mod motors besides the Shelby. It's a natural multigrade (no viscosity index improvers), has a bit more ZDDP than the diesel oils, a ton of soluble moly (friction modifier and anti-wear), and a 3.3 cP HTHS viscosity.

Compare that HTHS viscosity to the major 30 weights

Mobil 1 5W-30 - 3.13 cP
Mobil 1 10W-30 - 3.26
Pennzoil Ultra 5W-30 - 3.1
Pennzoil Ultra 10W-30 - 3.2
Amsoil 0W-30 (Signature Series) - 3.1
Amsoil 5W-30 - 3.2
Amsoil 10W-30 - 3.2
Red Line 5W-30 - 3.8

The 20-weights for comparison
Mobil 1 0W-20 - 2.6
Mobil 1 5W-20 - 2.77
Pennzoil Ultra 5W-20 - 2.7
Amsoil 0W-20 - 2.8
Amsoil 5W-20 (XL) - 2.8

Basically the more VIIs (polymers) in the oil the more compressible the fluid becomes under shear. Base oil polarity also plays a part in how much viscosity is retained under heat and shear, and PAO and group III are essentially non-polar, POE is highly polar.

I agree the a 5W-30 is preferable when dealing with off the shelf oils, but there are some exceptions...mainly the ester based oils like Red Line, Motul, Neo, etc.