Oil

It isn't far off
Kinematic viscosity of water at 68 F is a bit over 1 cSt.
vs
10W-30 at the same temperature is around 200 cSt
and a
20W-50 is around 550 cSt.

get the oil up to 210 F and a
10W-30 viscosity will be around 11 cSt
20W-50 viscosity will be around 18 cSt

but push that oil up to 250 F
10W-30 drops to 7 cSt
20w-50 drops to 12 cSt

Had an engine that would run oil temps over 260 F on long highway runs ( not coolant, just oil)
and yes it did get 20w-50

had another engine, a staight 6, that when fully heat soaked would start to loose pressure over 60 mph.
That engine was just worn! A high volume pump and thicker oil only marginally helped. But it kept running..:)

and yes, there is a range of viscosity allowable within each grade. You can get the info from most manufactures or send out for testing if its important enough.

For the original poster it seems to me he just needs to get in the ballpark.