OK...

Mixing a 10W-40 and a 5W-30.

For clarity, we should also consider a few other viscosities. And define what viscosity is. You never see a 5W-35, or a 7W-35. And mixing a 10W-40 with a 5W-30 won't give you a 7W-35.

The "30" and "40" are ranges and so... Login to read more.