Engine-Oil Temperature

Mike Critchfield

Green letter AThe traditional thinking is that the “normal” sustained operating temperature range of oil in diesels is between 210 and 230 degrees Fahrenheit when towing. It’s generally accepted that petroleum-based engine oil starts oxidizing (breaking down) around 240 degrees F (115 degrees Celsius), while full synthetics, such as Rotella T6, can handle temperatures above 300 degrees F.

Once oil starts the oxidation process, whatever damage is done by overheating can’t be fixed, much like getting a crack in a windshield. Run oil “hot,” and you’ll need to shorten up the oil-change interval.

That said, you should be OK if the synthetic-oil temperatures hover between 240 and 250 degrees F when towing in hot weather. Synthetic engine oils are designed for higher heat loads/applications and longer oil-change intervals.

While you are watching the engine-oil temperature, pay close attention to the difference between the engine oil and engine coolant (water). That delta should not exceed 15 degrees F, with the engine oil being the cooler of the two. If the coolant delta is greater than 15 degrees, that’s an indication that the exhaust gas recirculation (EGR) cooler is plugging up and not cooling efficiently. When engine-oil temperature is just above the boiling point of water (212 degrees), moisture buildup in the crankcase is minimized.

It should be noted that modern diesel computers will activate a Check Engine Light (CEL) warning that oil temperature is too high, and even put the engine into a protective mode.

Engine-Oil Change Intervals

Mark Loftus

Green letter AIn my opinion, using synthetic engine oil in a diesel that is used to tow an RV is never a waste of money. Synthetic engine oils withstand the rigors of high heat and the pressure loads in diesel applications far better than non-synthetic (petroleum-based) engine oil.

You can use the truck’s message center as your oil-change interval guideline or the schedule noted in your vehicle owner’s manual as listed under “Severe Use.”

An even better indicator is having your Duramax’s synthetic oil analyzed by an oil lab. Pull a sample at the next oil change and have it checked. That will give you an idea if the oil needs to be changed sooner or later. Such analysis will let you know the amount of contamination, wear rates and the overall condition of the engine. It’s like going to your doctor for blood work.

I’ve used Blackstone Laboratories a number of times over the years for oil-sampling service.

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  1. I was glad to see the information on oil temps for diesels when towing. I would go a step further with a question. I have a 2009 F-450. What is the temp range on the transmission? In 95-degree heat and towing six to seven hours with some steep grades, I was in the 200- to 205-degree range.

  2. When discussing “delta,” the difference in temperature between coolant temperature and engine-oil temperature, I think you meant to say that engine coolant temperature should be the cooler of the two, not engine-oil temperature.

    • Bruce W. Smith replies: You are correct, Roy. The coolant should be cooler than the engine oil, the reason being engine-oil temperature varies a lot in any motor depending on workload.

  3. My oil temp is running 210 -230 my coolant is running 208-219 what could make the oil temp run higher? I have a 2017 dodge ram 2500 I use rotella oil. I’m always hauling with my truck up to 10,000 lbs

    • 2014 Dodge 3500 6.7 Aisin CC Dually 4×4. Running gear all Amsoil. My oil is doing the same 204-231. Only the last few weeks. Rotella 15W-40 and one quart of Hyper-Lube every 5k For the last 192k. It is hot outside, but same as any other year. Coolant and fans are keeping it from rising any more, but I feel like its just too warm. Maybe clogged oil cooler?

  4. Maybe bad thermostat? I haul 15-30k most times. Nothing deleted on this one yet. All stock Except aftermarket intake filter and baffle removed.

  5. I have a 2003 f350 with the 6.0 in it and my oil temp hit 240 degrees going 65 mph with out hauling a load. Do you think an engine flush would help or does it need a new oil cooler?

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