Hell, multi weight oils start out at their low number and get more viscous as
their temp rises.

no they don't. 15W-40 is the same viscosity ("thickness" if you will) at "room
temp" as straight 15W oil is, and the same viscousity as 40W at engine
operating temperature. 15W-40 is much more viscous at room temp than it is at
engine operating temp. If you doubt that pull the dipstick on a warm engine
and see.