OCTOBER 2019
Engine-Oil Temperature

Red letter QI really appreciate the Diesel Tech Q&A addition. I have a question about my 2012 Ford F-350 6.7-liter diesel. There is an oil-temperature gauge, but no one can tell me what the upper limits should be and for how long. I have come up to 230 degrees while towing up a mountain grade, but it quickly drops to the 200- to 210-degree range when the grade levels off. The coolant temp never seems to get above the normal range. I use Shell Rotella T6 full synthetic 5W-40. I use the gauge to let me know when the oil is warm enough to begin towing.
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

Red letter QI have a 2013 GMC Sierra 2500HD with the 6.6-liter Duramax. At the last oil change, at 72,000 miles, I opted for 15W-40 Mobil synthetic at the advice of the shop. Does synthetic oil last any longer than regular oil in diesels, like they say it does for gasoline engines, or should I follow the Change Oil Soon message? Is synthetic oil a waste on diesels?
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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Bruce W. SmithA respected automotive and RV journalist and longtime Trailer Life contributor, Bruce W. Smith has held numerous editorial titles at automotive and boating magazines, and authored more than 1,000 articles, from tech to trailering. He considers his home state of Oregon a paradise for RVing and outdoor adventure.


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