Hi,
MrWideTires - The lighter - thicker debate has been beaten to death here but this may interest you;
For 1941 Model US Cars
Oil grades for all Makes and Models
The most common recommendations for in-line 4, 6, I8 and V8, V12 follow;
Engine:
Over 32F = 20 or 20W (highest 30), then some 90F> 30, 40 (highest 50)
Over 10F = 20W (lowest 10W)
Over -10F = 10W
Below -10F = 10W (plus 10% Kerosine - for all vehicles except Crosley)
Gearbox in Summer = 40, 50 , 70, SAE 90, SAE 90 EP, SAE 140 (1)
Gearbox in Winter = 30, 40, 50
Automatic = Hydra-Matic fluid
Rear Axle Over 0F = 80 EP, 90 EP, 80 Hypoid, 90 Hypoid, 140 EP
Rear Axle Below-0F = 80 EP, 80 Hypoid
So low viscosity oils really are not a new thing - they were a factory fill in 1941!!
The modern low viscoity lubricants are performing as intended in their correct applications - both in NA and here in OZ too!