i'm trying to imagine what happens to oil viscosity over time as the car warm up to temp. corrent me if i'm wrong: 1) with a multi grade like 10w-30, it acts as lower viscosity at lower temps, and it moves up to 30 grade. 2) then as it heats up more it thins out 3) depending on the generated heat, the viscosity probably goes up and down within the grade limit. i made a graph to illustrate my view: let's say ambient temperature is 40F. what do you think?