Brad Penn 10-30 vs 20-50, Weight vs Temperature Options

[1988Hawk]

On a 88 degree day in Chicago I have tested two oil weights and have gotten the following results:

1. Brad Penn 10-30W, with peak running temp 205 degrees.
2. Brad Penn 20-50W, with peak running temp 225 degrees.

Both exhibited acceptable pressure, so the question is which one would you select for summer driving???

[scott_in_nh]

Obviously I have no basis for a discussion with Jake on what is the correct viscosity for any given engine from a lubrication standpoint.

But to the discussion about viscosity vs. temperature I have some expertise (I’m an application specialist for fluid based cooling – chillers and laboratory bath circulators).

Specific Heat – how much energy it takes to raise 1 gram of a substance 1 degree Centigrade, measured in Calories (which can be converted to watts). The higher this number the less the temperature goes up for every calorie of heat absorbed, so a higher number is better. I did a quick look around and it is not clear whether lower rated viscosities have higher specific heat, but they might.

Thermal conductivity - is the heat energy transferred per unit time and per unit surface area, divided by the temperature difference. I could find comparisons among oil of the same specified viscosity (20w-50 in this case), but not for different viscosities. Again, lower viscosities might have better thermal conductivity.

Viscosity – Informally, viscosity is the quantity that describes a fluid's resistance to flow due to the friction between its molecules. As mentioned, higher viscosities flow less at the same pressure than lower viscosities. Viscosity can also affect the type of flow through an oil cooler, possibly forming a boundary layer at a higher viscosity that insulates the hot oil further from the internal surface of the cooler.