Ron:
You're missing it. 10W-30 means the viscosity is equivalent to what a 10 weight would be at 0 degrees
and equivalent to what a 30 weight would be at 212 degrees. The 10w-30 does get thinner as it gets hotter, but at a lesser rate than a straight weight
oil would. Even 30 weight
oil is pretty watery at 212 degrees.
Bob