HTHSV - High Temperature, High Stress viscosity.
The two 0W-20 oils I mentioned are a lot lighter than 5W-20 oils.
Adding some 0W-40 simply increases the viscosity enough so that at operating temp's it more closely matches the viscosity of a typical 5W-20 while still being much lighter on start-up due to the 200+ viscosity index. In other words your just making a somewhat heavier
0W-20 oil while still maximizing the full saving benefits.
The two 0W-20 oils I mentioned are a lot lighter than 5W-20 oils.
Adding some 0W-40 simply increases the viscosity enough so that at operating temp's it more closely matches the viscosity of a typical 5W-20 while still being much lighter on start-up due to the 200+ viscosity index. In other words your just making a somewhat heavier
0W-20 oil while still maximizing the full saving benefits.