Mixing different weights of oil

[RalphVa]

Why not use 5w40 all year round? Even 5wxx is heavier at startup than at operating temperature. 0w40 would be even better year round and would STILL be heavier at startup than at operating temperature.

Ralph

[DieselPower]

Mixing different oils isn't suggested even if it's the same manufacturers oil. Different viscosity oils even from the same manufacturer and "brand" can have different additive packages. When you start mixing already blended oils your throwing out millions of dollars of research that go into blending and testing specefic oil blends. Mixing base stocks and mixing final finished blends is completly different. If your mixing base stocks here's a handy calculation tool Viscosity Index Calculation page