noticeable difference in oil viscosity.

[RobW]

some may remember that raising oil pressure thread a few weeks ago where I was surprised some were using 10W-30 or 10W-40 in 472 and 500 engines in hot humid climates. I've been using 20W-50 for years with no issues. However it was suggested in that thread that the heavier oil would actually cause the car to run hotter and put more strain on the engine.
So this past Thursday evening I changed oil and filter from 20W-50 to 10W-40. On the ride to work in the morning about 13 miles car ran fine with the A/C on and no traffic. Coming home sitting in traffic with temps in the high 90's the car started to get hot. Not like it was going to overheat but more like a carb heat soak issue. I could hear and feel the gas almost boiling. So changed oil again back to Castrol GTX 20W-50 and problem solved. Car has 94,000 miles and just recently did the heads, valves, valve seals and guides, timing chain, and cleaned the oil pump and passages which weren't clogged anyway.
So for whatever reasons my 472 likes the heavier oil and runs smoother and cooler.

[TJ Hopland]

Interesting.  I wonder why?   Does thicker oil transfer heat better?

[Jeff Rosansky CLC #28373]

Does that Castrol have zinc?
Jeff

[RobW]

Both good questions. Don't know but I will try to find out. It was just so noticeable. Strangely the week before it was extremely hot and humid here with real feel temps over 110* and this didn't happen in traffic.

[savemy67]

Hello Rob,

Your profile indicates you are in New York, where, like Baltimore (where I am), the humidity creates a "real feel" heat index in excess of 100 degrees on those sultry summer days.  As humans who depend on evaporation of moisture from our skin to regulate body heat, the greater moisture content of the air surrounding our bodies on humid days degrades the cooling effect of evaporation.

While humidity may effect how we humans feel, it does not have the same effect on car engines.  For cooling purposes, generally the more humid, the better.  In terms of air, the water jackets of an engine far exceed 100 percent humidity, but this is ideal for transferring heat (energy) away from the cylinders.  In other words, the density of a fluid determines the heat transfer capabilities.  The denser a fluid, the more closely packed the molecules are, and the closely packed molecules can more easily transmit energy, one molecule to another, than more loosely packed molecules.  This is why air is a good insulator.

The answer to TJ's question is yes and no.  The reason is that oil decreases in density as temperature increases.  At normal engine operating temperatures in the neighborhood of 210 - 225 degrees F., the dynamic viscosity difference between SAE grade 40 and grade 50 oil is effectively negligible.  The kinematic viscosity difference also starts to converge as kinematic viscosity is the ratio of absolute viscosity to density, so as density decreases with a rise in temperature, kinematic viscosity also decreases.  At some temperature not too far above which most car engines run, the difference between SAE 40 ans SAE 50 oil ceases to be a factor.

Where the higher grade oil is a factor is in those first few minutes when a cold engine is warming.  This is where the more viscous (viscosity is a measure of resistance of the molecules of a fluid to move past each other) grade 50 oil would keep more of the oil clinging to the bearing surfaces than the grade 40 oil.  If you think your engine needs grade 50 oil when it is below operating temperature, then you may have excessive bearing clearance.  If you noticed a difference in how your car ran at normal operating temperature - as described in your post - I think there are too many variables to reach a definite conclusion.

As long as you change oil and filter regularly, you will probably get many miles of happy motoring out of your engine.  If you do a lot of winter driving, I think the worst that could happen with the use of grade 20W oil as opposed to 10W is that you  might put a little more strain on your starter.  My Alaska pipeline buddies used to light fires under the Caterpillar equipment in order to get the fluids moving when t was -45 F.

Jeff, I do not know if the Castrol contains zinc.

Respectfully submitted,
Christopher Winter

[fishnjim]

Christopher explains it well, but I'd add, as asked, heat transfer decreases with increasing viscosity but viscosity decreases with increasing temperature.  [Shear is another story - not so simple.]   
So you could have had exactly the same viscosity at moments in time with both oils and not even known it.  How's that for confusion?  {For that reason, I'm not even going to touch zinc.}  Actually higher viscosity will cause more heat as lost work, because the motor has to do more work to pump the thicker oil and make it harder to remove.   But it's negligible as said above.   Cooling systems have some safety factor.
Lots of confusion on SAE viscosity numbers.   Test methods are designed to magnify difference when small.   eg; Difference between 40 and 50 is not as big as it appears compared to 10 or 20.
I doubt seriously the oil had much to do with your experience, but in your mind, as others, prefer the higher visc and there you will remain.  Self anecdotal evidence reinforces those thoughts.   At that mileage, you're lucky if you're getting lubrication and cooling (at all) and not just kidding yourself.   
If it works, do it, but you always stand to suffer consequences both seen and unseen.   
If I have overheat issues, I check my cooling system, not my oil.

[RobW]

"At that mileage, you're lucky if you're getting lubrication and cooling (at all) and not just kidding yourself."

Meaning what? Just idling in bumper to bumper traffic with little to no air flow? Yeah that's entirely possible. That's assuming you mean mileage as in miles per hour. If you mean mileage as in the car having 94K on it all I can say is when I tore down the top half of the engine to change valve seals everything was extremely clean. Also did timing chain and removed oil pan to clean oil pump, pickup tube and screen. Also cleaned oil passages but again nothing caked up and nothing clogged. The only reason for the cleaning was I found a few small bits of Teflon sitting under the valve covers. Was most likely from the valves and not the timing chain as those Teflon gears were intact.