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Message: [QUOTE="TARM, post: 1010726, member: 578"] Yes I do understand please do not assume I do not. Why in the world would you want to cool a system @ 100 degrees first of all? LOL I think you must have meant 200 degrees (easy typo as I am full of those LOL) But yes I understand what you are trying to say. But that is one of the main points of a T-stat, is it not? To prevent extra cooling to allow the engine including the oil to get to it proper operating temp quickly and before cooling. We want the oil to get up to 200 degrees ( 212 actually would be ideal). Then the coolant which would be very close to ambient temp in the radiator starts to flow into the engine. The system on these trucks have huge cooling capacities. I think its understood that we want the oil to get up to temp as quickly as possible The engines coolant system in every properly running engine I have seen gets up to t-stat operating (opening temp) temp long before the oil does. This is actually by design as it helps bring the oil up to temp quicker via the two way transfer of energy in this case heat between the two fluids. Again these system are spec'd to run where oil temps should never be over 20 degree of coolant temp. If it is not ( is larger) then there is an issue with the cooler. If the spread is met then what is the point? It means everything is working correctly in terms of coolant to oil temps. Now if they are both too high say coolant 240 oil 260 ( just as an example) Then this is an issue with the cooling system not the oil cooler. Something is preventing the cooling of the coolant i.e stuck Tstat, blocked radiator, fan clutch etc It does make your (my) head go in circles as you flip things around and around think about them. It certainly does mine LOL The only time what you are saying could come into play is if you have a system where the oil temp in ratio to the coolant temp is outside that range of 15-20 degree maximum but it would have to be below the full opening of the T-stat temp. It would means the oil cooler for whatever reason was not keeping up with the cooling of the oil at the typical temp spread and so its getting hotter and hotter yet the coolant temp is some how staying at a temp that is below its full open. The reason it would have to be below fully open or only doing this during short peak load event is that even if it did start to cool it a bit sooner such as using a lower rated T-stat say180 by opening sooner eventually in a steady state condition such as towing down the highway for say an hour or even 30 mins or less it would eventually over take any head start opening early gave it. Once its fully open its fully open. So if its peaking way over the 20 degree spread with a 195 or 203 when they are fully open eventually its going to do it with any or even no Tstat at all. Thus for this to apply it would have to be with a scenario where the tstat was staying in a range were it was open but not fully yet oil temps were still exceeding the 20 degree spread. As an example lets say its doing this with a 195 but at temp were the 195 is not even fully open say 205. So basically the oil cooler does not have the capacity to transfer enough heat to the coolant to drop the oil temp enough to keep it within the 20 degrees and that oil temp is already too high. Yet its coolant side cooling is enough to keep the coolant at that temp steady without going up. So lets hypothetically say that we had and engine that the oil cooler just was that way or say it was perfect and something like a defect in the oil passages etc whatever take your pick was causing oil to be hotter, creating a spread larger even at cruising speeds. So say you are seeing oil temp of 250 with coolant temps of 205 degree running a 195 degree T-stat. OK so we can see that the coolant temp is still below the level of even being fully open yet the oil temp is still too high. We have gone thru everything in the engine we can without pulling the damn thing and we just want it to keep running and keep oil temps in proper range. Oil pressure seems good and flow rate is fine also ( obviously if passages were somehow restricted this may have other issue in terms of lubrication etc but lets assume its all OK) In that situation yes I fully agree going to a lower T-stat opening makes complete sense as something to keep things in range. It will give more coolant and and thus a even larger temp difference across the oil cooler and help drop those temps lower. But again that is certainly the very rare exception to the general state of things with 7.3s. And if we are going to be debating the odd man out there we could basically make exceptions for most everything. But yes I would agree that in those cases one fix would certainly be to use a lower rated T stat to keep oil in its proper range. The only possible down side would be low heater performance and I will take keeping my engine running over a a bit cooler heat coming out of my vents. But this is again certainly not the norm or the way the 7.3 was oil cooling system was designed to perform in relation to the coolant system. All the various engine have a oil to coolant temp range spread. The 7.3 20 degrees I think the 6.0 and 6.4 is suppose to be 15 degrees and I am not aware of what the 6.7 is but I would guess close to the same or maybe they have some sort of electronic t-stat I honestly do not know. All of these engines are suppose to hold this ratio irregardless of the temp and its the T-stat that controls what the actual temp of that 15 degree spread is. THere is nothing different about the 7.3 in terms of this function. [/QUOTE]