Maybe a couple graphs of oil viscosity vs. termperature will make this clearer.
Here's four 40 wt oils. One straight weight and three multi grades.
Notice they all have about the same viscosity at 212 * F.
View attachment 1715599102
Lets say your oil is running at 140 *F (60 C).
At that temperature the viscosity of Brad Penn straight 40 would be around 60 centi-Stokes
while the viscosity of Rotella 15W-40 would be around 50 cSt and the T6 5W-40 thinner yet at 40 cSt.
Get those same oils up to 212 F and the viscosity of all are around 15 cSt.
This next one shows the difference in viscosity between some common multi-weights.
Notice the big difference in viscosity between 10W-30, 10W-40 and 20W-50 at 212 F
All brands will be similar to these. (These happen to be Brad Penn's viscosity data because it was easy to access and I use their oil.)
View attachment 1715599104
*(I know weight is not technically correct but its the term used in normal conversation, so I'm using here.)