OK, a little while back, I thought that mixing 0W-30 and 0W-40 in equal amounts would give you a 0W-35. Well, now I realize that the N and M in NW-M oil represents a range, not an actual viscosity. So, instead, let's look at the actual viscosity of, in this case, 0W-30 and 0W-40 Mobil 1. From their website tech data at 100 degrees Celsius 0W-40 viscosity = 14.3 cSt 0W-30 viscosity = 10.3 cSt now if you combine these in equal amount, assuming that the resultant viscosity can be interpolated based on the amounts of each component oil, you'd get in this case (14.3 + 10.3)/2 = 12.3 cSt. Now, looking at the handy-dandy viscosity/grade chart on the BITOG front page, an SAE 30 grade oil has a viscosity range of 9.3 to 12.49 cSt. So in this case, we'd have a mix that is still a 0W-30, albeit on the heavy side. Nice summer brew, perhaps? Especially since UOA's tend to show M1 viscosities dropping after a few miles (then thickening back up again, according to some). So, for a mix of 3 parts 0W-30 and 1 part 0W-40, we'd get (3*10.3 + 14.3)/4 = 11.3 cSt, just thicker than the middle of the 30 weight range. Good winter brew? A valid competitor for the much touted but oh-so hard to find German Castrol 0W-30? OK... is this a better way to look at mixing viscosities? Discuss!