The most common cause of over heating is a defective cooling system. Refer to cooling-system trouble shooting (par 89). Other possible causes are the following:
[1] 0il with improper viscosity. (par 201).
[2] Too lean a fuel mixture (par 55, 56).
[3] Restricted carburetor air cleaner (par 50).
[4] Late ignition timing (par 24).
[5] Radiator restricted by bugs, weeds, or trash (par 95).
The above is from Automotive Trouble Shooting For WW2 Vehicles, Volume 1.
Cause number one goes along with what I have said for long time and that is you should use the viscosity specified by the manufacturer of your engine. If the original designers say to use 30 weight, they didn’t mean for you to use 20 or 50 weight oil. This doesn’t mean you can’t use modern multi-weights like 10w30 depending on the needs of your vehicle. This is a good combination oil because you don’t have to worry about outside temperatures. In other words your vehicle is protected above and below 32 F. If you expect to drive below 0 F then you will want to revisit the charts. Use the weight of oil that your vehicle’s engine was engineered for.