DECEMBER 2019
Oil Change Intervals and Analysis

Red letter Q

I have a 2016 Chevy 3500HD Duramax diesel. It has 60,000 miles on it, and I’ve been using Shell Rotella 15W-40 oil since I’ve had it. Can I change over to synthetic oil now without it hurting the engine? If so, what would you recommend?
John E. Gunnels

Red letter QI have a 2005 GMC 2500HD diesel, and it has 150,000 miles on it. I have been using Amsoil 5W-40 Max-Duty synthetic diesel oil. My tech says I should have no issues running 10,000 miles between oil and filter changes. Do you agree
Darryl Wolter

Green letter A

Engine oil in diesels has the burden of not only lubricating the engine but doing so while being contaminated with carbon and fuel. So it’s critical that whatever oil is used meets or exceeds the engine manufacturer’s requirements so the additives in that particular engine’s oil provide the best lubrication for that application.

The same holds true for oil-change intervals. At minimum, the oil-change intervals should be done in accordance with the type of use recommended in the vehicle’s owner’s manual. For those towing trailers or hauling truck campers, that means changing oil under the “Severe Duty/Severe Use” maintenance schedule.

Whether or not to use a full-synthetic or a synthetic-blend oil (a mix of synthetic oil and conventional oil) is more of a personal preference. Either will provide engine protection as long as it meets or exceeds the manufacturer’s recommendation.

See Related
Diesel Tech Q&A: October 2019

In general, full-synthetic engine oils will provide better protection in extreme temperatures (hot or cold) than synthetic blends because full synthetics are formulated using higher performing additives. That’s why full-synthetic oil costs more than synthetic blends, which cost more than conventional engine oils. The personal choice comes down to how much you want to spend on oil changes per year and over the life of the truck.

It’s generally accepted that it’s OK to switch between synthetic, synthetic-blend and conventional engine oil, according to the major oil manufacturers. For example, Mobile Oil’s consumer tech forum says, “Switching back and forth between full synthetic and conventional oil will not damage the engine. Of course, this depends on the current engine condition and the quality of the conventional oil being used.”

How often the oil is changed should be based on use and the environment in which the truck is used, not necessarily on the brand of the oil.

Professional Oil Analysis

The best way to determine when oil should be changed is to start with the engine manufacturer’s recommendation, and then change that interval, up or down, from data gathered using a professional oil analysis from one of the labs around the country. (Note that vehicle oil-change monitors don’t actually test the engine oil; they use algorithms based on how the engine is being operated.)

To really know if an oil change is needed at, for example, 3,000, 7,500 or 12,000 miles, the oil should be analyzed on a periodic basis. Such testing will detail the “health” of the oil at the time it was changed, and if it could go longer or needs to be changed sooner. Relying on what the label on the oil container claims for oil-change intervals or what a mechanic says is a bit foolhardy. Extending oil changes on a diesel beyond 10,000 miles may not have an effect in the first 200,000 miles, but it may mean the difference of having to spend $12,000 on a complete engine rebuild at 250,000 miles instead of 500,000, or needing to replace smaller engine components sooner rather than later because they were not being lubricated properly.

My philosophy on a diesel that is used less than 20,000 miles per year and is still under warranty is to use a synthetic blend such as Shell Rotella T5 Ultra or an equivalent oil that the engine manufacturer recommends. Then I’d initiate an oil-analysis routine and use that data to establish future oil-change intervals.

On diesels that log more than 20,000 miles a year, especially those used in the southern states, Alaska and Canada, I’d probably use a full-synthetic such as Rotella T6, Valvoline Premium Blue, Amsoil, Castrol Edge or Mobile 1, and would definitely use oil analysis to determine what oil-change interval is best for the way the truck is being used.

Oil analysis is cheap insurance in the overall scheme of diesel maintenance. It can save money in the long run and provides a great insight into a diesel engine’s overall health, much like a blood test shows potential problems within your body. A good lab analysis done at various intervals can track the accumulation of wear particles, soot and other combustion byproducts in the oil to help you decide when to change the oil.

You can find labs that test diesel engine oil via an internet search. One such company is Blackstone Laboritories. Blackstone provides free test kits, and charges $28 for oil analysis.


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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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