Oil/Heat question

This is not a discussion about the best oil. We've had enough of those and don't need more. I just need help from someone that knows more about oil than I do (not hard). The question is, if we assume the oil is the same weight, can oils from different manufacturers have significantly different thermal properties? Last weekend I started a thread about my coolant heating up quicker than I feel it should be. I mentioned I'm seeing similar heat cycling in my Cayenne. Today I took the Cayenne out for about 40 miles each way mostly highway (no towing and no load) and its oil got significantly higher than I've seen it before. The hottest I've seen it get before was 217 when I had been flogging it hard on a hot day, but today was relatively cool and mostly highway cruising and its peak was 228 (on the highway, it dropped to 218 in the city). I saw similar results on the return trip too. Both the cars had their oil done in the last 1000 miles and both were switched to Motul. The 996 has used M1 until now (68.5k) and the Cayenne used Penzoil starting with its first OCI (19k total, 14k on Penzoil). Blaming the oil doesn't feel right, but I don't know anything about the thermal characteristics beyond that it does serve a degree of cooling function. The brand of oil, however, and recent change is one of the few similarities I can tie between two vehicles that appear to not be performing as they had previously.

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Very funny. I applaud your valiant effort.....

Strictly speaking there should be very little difference in thermal properties, like heat capacity and thermal conductivity. What can differ by quite a bit is viscosity at temperature. The more viscous the oil, the thicker the "boundary layer" is at heat transfer surfaces (e.g. the walls of the oil/water heat exchanger). Thicker boundary layers slow heat transfer and hence the bulk oil temperature will rise. One of the primary reasons some oils protect engines better than others is that they don't thin out as much at temp. HTHS readings are one insight into this, and they can vary considerably between oils of the same weight (e.g. 5W40). Take a look into the HTHS values for the various oils you've run and are running to see if they correlate to your engine oil temperature observations. I suspect you'll find your answer there. There's my $0.02. (Gee, my chemical engineering degree can sometimes still be useful!)

http://www.hddeo.com/hthsarticle.html High Temperature High Shear Viscosity of Engine Oils What it Means to Your Engine High temperature high shear (HTHS) viscosity of engine oils is a critical property that relates to the fuel economy and durability of a running engine. The drivers behind lowering HTHS viscosity are new global governmental regulations to improve fuel economy (FE) and lower greenhouse gases (GHG) in new vehicles. Lower HTHS viscosity tends to improve FE and lower GHG but higher HTHS viscosity affords better wear protection so a careful balance must be found when formulating an engine oil. Sufficient HTHS viscosity is critical in preventing engine wear in the critical ring/liner interface area by maintaining a protective oil film between moving parts. One method used to measure HTHS viscosity is ASTM D4683. Oil is introduced between a rotor and a stator at the test temperature of 150�C. The rotor experiences a reactive torque to the oils resistance to flow (viscose friction) and this torque response level is used to determine the HTHS viscosity. HTHS viscosity by ASTM D4683 has been found to relate to the viscosity providing hydrodynamic lubrication in light duty and heavy duty engines. HTHS viscosity has also been found to relate to fuel economy. Think of the protective oil film as if you are trying to swim. If the film is too thick like molasses you can barely move and have to expend a lot of energy; too thin and you sink to the bottom. What you want is the right balance of support and ease of movement. The oil has to be thick enough to maintain separation of the critical moving parts but thin enough to allow for fuel efficient operation. A new heavy-duty engine oil category, PC-11, is currently under development. PC-11 may be split into two categories one with HTHS viscosity less than 3.5 and one equal to or greater than 3.5. PC-11 oils with HTHS viscosity <3.5 would offer FE benefits but would be restricted to engines designed to run on lower HTHS viscosity oils. This may exclude many older engines. Engine manufacturers are evaluating their hardware to see if engine durability, especially for ring and liner scuffing is an issue with low HTHS viscosity oils so they can take advantage of the engine lubricant delivering improved FE to meet the new US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA) FE and GHG requirements. Engine manufacturers may have to redesign their engines to take advantage of the potential fuel savings of low HTHS viscosity oils. PC-11 oils with HTHS viscosity equal to or greater than 3.5 would cover the heritage fleet and new engine requiring higher HTHS viscosity for wear protection.

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By gum who would have thowt..

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Unless you switched to using some 10w-50, 10w-60, or 20w-50 oil I doubt the oil is the reason for the temps are you seeing. Even then I'm not sure it would cause the oil temp to vary all that much. (FWIW, compared to 0w-40 oil, with 5w-50 in my Turbo the hot idle oil pressure is a bit higher with 5w-50 which if the thicker oil caused the oil to run hotter would I believe not be the case.) Have you checked both radiator fans are working as they should when they should? Have you checked the radiator ducts for trash buildup? The temps could be normal. The oil generally runs a bit hotter than the coolant. But you must be sure both vehicle cooling systems are fully functional.

Thanks for the info guys. Macster, as I said, blaming the oil doesn't feel right. I'm just trying to find similarities between two vehicles that aren't acting as expected but in a similar way. The 996 is clear from the front, but the bumper still needs to come off to check between the rads. Everything else on the system appears to be functioning as expected. The Cayenne is a bit easer to do an inspection of (though oddly harder to clean the crap from the front of the rads) and its clean too.

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Any proportional relationship between HTHS and specific heat capacity? Also, what does this mean regarding new engine spec tolerances designed to run on HTHS oils?