"I can see where it might make a noticeable difference if you have a fresh air heater. "

I think the 1951 truck (JD's year) had either an optional/accessory Recirculating Heater or a Fresh-Air/Air-Flow heater?

The Fresh-Air/Air-Flow heater (from Jim Carter) had a coolant flow valve (a "Ranco Heater Control Valve") mounted on the engine side of the firewall. (the interior part of the valve is shown in a photo in JC's article)

The Recirculating Heater had no coolant-flow valve.

I may be wrong - if so, please correct me.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.