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???

The thicker oil ran hotter?

Correct.

Thicker oil is harder to pump, costs horse power too…

Not sure this is an answer but find it interesting. Tony at Translog GT afterhe did the complete engine rebuild on my Teener, and after the break in,, changed the break in Oil afer 500 Miles with SWEPCO 15W40.

My own theory is that the lighter weighted oil dissipates heat more effectively. I have seen this in past years when switching oil, when I first used BP 20-50 there was a noticeable increase in temps.

I will never join the thick oil mindset.

...BTW I'm now trying Delo 400 15w40, all good so far and it's cheap.

Brad Penn can also be had in 15W-40.

I had similar a experience. I also suspected that the thicker oil did not dissipate the heat fast enough. Which cools faster. Molasses or Water.

I tried both and noticed the same thing the first summer I had my car.
I've been using the BP 10w-30 in NH for about 20,000 miles over 3 years, including AX, with no issues.

Likely a combination of the thermal properties of the oil and reduced flow through the oil cooler - a fluid of the same density but a higher viscosity will have less flow at the same pressure.

It all bolus down to the internal clearances of the particular engine. There is no general response for this question, as after so many years most every engine has been internally goofed with and clearances can be all over the place.

My engines use 15/50 oil best in hot climates, but thats because the internal clearances are set up to be optimized with this oil grade.

Oil pump specifications and materials also play into this, and are often not considered when selecting an oil grade.

I've used Mobil 1 15W50 since my engine was built 10 years ago. I went to a lighter viscosity once (Royal Purple 10W40) and I kept getting a low oil pressure light. The Mobil 1 works great for me and the bottom line is to use what ever works best for your engine.

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.

Exactly. I have carried out years worth of development for and with Joe Gibbs Driven Racing Oils to study these effects with different base oils and additive packages in European engines. This is limited to Porsche, but also includes MB and BMW, and even SMART engines.

Thick oil often builds more heat, unless the internal clearances of the engine favor it, and unless the characteristics of the oil allows it to not only collect, but also shed heat.

At this level we use cinestoke as the unit of measurement, and its well beyond what an oil is rated at only at 212F, as directed by the API.

My record is 360 chassis dyno pulls in a week, and 13 test oils. The ONLY goal with those tests was to gather trend data to support the changes the engine saw in regard to oil and engine temperature with the different oils.

No general statements can be made, the variables from oil to oil and etc are too great. I am most concerned with HTHS data and the oil's ability to maintain pressure at idle, when at elevated temperatures. This is NOT to be confused with viscosity, as viscosity index comes into play at temperatures well above the API 212F test.

We use Centistoke to measure viscosity as well. For the nerdy among us who would like to know (but don't already) there are two types of viscosity:

Dynamic viscosity - measured in centipoise [cP]
dynamic viscosity measures to force required to shear the fluid

Kinematic viscosity – measured in Centistokes [cSt]
Kinematic viscosity is the dynamic viscosity divided by density

So in other words if you have the same volume of two liquids that have the same "thickness" but one of them weights more, then it will have the same Dynamic viscosity but a higher Kinematic viscosity.

The higher Kinematic density (Centistokes) means that it is harder to pump.

EDIT: my mistake, I thought jake made a typo, cinestoke is also a unit of measurement for viscosity = unit millimeters squared per second.
I am not familiar with this one, maybe Jake can fill us in.