Cummins oil change - synthetic?


New member
OK, another newbie question in several ways. While far from being a newbie mechanically, this is not only my first experience with the RV life (fulltime), but my first experience owning a diesel truck (although I've been around diesel tractors and heavy equipment all my life).

To boot, this is also my first experience with synthetic oils, so I felt like the safe bet was to throw my question out there. I just traded for a 2004 Dodge 3500 Turbo Cummins with 77,000 miles. The truck has been run on Rotella T 5W-40 and the prior owner changed it every 10,000 miles. Now, I've always been pretty dogmatic in my old school ways with gas engines and conventional oils to change at 3,000, but I know it is both unnecessary and expensive to do so with the new oils - especially so with the synthetics and the 12 qt oil changes on the Cummins!

So, do you have a recommended oil change interval? I read this article recently - The Dodge manual recommends changing at 7,500 under low load applications. How is that number affected by going to synthetics? I figure on going ahead and changing it for my own baseline (the prior owner said it was about 10k in, anyway), but I wanted to poll the forum. I've been reading some about oil analysis. This seems especially popular with the pushers, but do you believe it's worthwhile in my application?
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Dirt Rider

Active member
I have 2 cummins powered trucks 220K and 250K, change the oil at 3k, even though it holds 3 gallons, the oil gets polluted and black pretty quick, unless you put on a bypass oil filter I would not go 10k. Rotella is a good way to go.
It is ridiculous to change oil in a Cummins every 3k miles. The OEM suggests every 10k as a minimum.
In my Mercedes 906LA, a similar sized engine with similar output but a 30 qt pan, oil analysis at 25k was outstanding after a trip from Anchorage to Mongolia and back. Using high sulfur fuel for over half the trip.
Do oil analysis if you need reassurance, it is only about $35.

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Dirt Rider

Active member
I think you need to figure in the duty cycle when determining oil changes, are you pulling a trailer up hills? extreme four wheeling? What are you putting it through? I put mine through severe duty and need it to be 100% reliable.


Viking with a Hammer
True. But keep in mind that light duty is terrible for oil as well. My personal trucks 6.2 only has 13,000 miles. I change the oil every 3000 to keep that watery white shmegma out, that comes from condensation. Time is a monster, as much as towing is.


2006 Dodge with a Cummins in it, bought with 115k, now with 160K. Change every 7000-9000 miles with Rotella T5 and Donaldson ELF7349 filters. I have done MULTIPLE oil analyses with Blackstone Labs. Oil, while black, is reported to be in excellent condition upon removal, with the suggestion from them I could get more out of it if needed.


Expedition Leader
I have 2 cummins powered trucks 220K and 250K, change the oil at 3k, even though it holds 3 gallons, the oil gets polluted and black pretty quick, unless you put on a bypass oil filter I would not go 10k. Rotella is a good way to go.
If you do a Blackstone or similar oil test you see that the oil isn't polluted. It turns black in a block but my results are nearly identical to my old 1998.5 5.9. Synthetic can induce seepage on some engines. I run Delo's 15-40 C/K and change every 6K or so. I don't use any measurable oil in that period. Engine's got 95K on it.


New member
The whole synthetic induces leaks/seepage is an old wives tale...

Sort of, I thought the same as well.

It was a real problem, there was something either in or missing from (I forget what I was told it was) early synthetics that did cause issues with seals.

That was fixed 30 years ago or so and hasn't been an issue since so nothing to worry about today.

I have almost exclusively ran forced induction vehicles for the past 20 years, so everything gets synthetic. My TDIs went 10k, my old 12v went 7500-10k and my 4bt now is sort of just once a year (could be 10 miles could be ,10k miles). I've even started getting most of my customers on cheap synthetic and going 7500-10k, the couple bucks more for synthetic to go 10k is less than 3 conventional oil changes at 3k. Better for the environment, their wallet, and my sanity.

The 4bt was on rotella t4 or 5, I forget, it sat a few years. It's on T6 now, back to regular use and will be on 10k intervals. I even run T6 in 2 of my gas turbo cars.


Viking with a Hammer
The seals were wasted. Crappy old oil didn't have the detergent to wash away the dirt that was keeping the seal from leaking. Synthetic did, and leaked very much.

Made tons of money from that. Better yet is the piss poor red extended life coolant. That stuff has wrecked many coolant systems and has become a huge cash cow for me.

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Engineer In Residence
All rubbers undergo whats called hydrocarbon expansion when exposed to oil/fuel. This is accounted for in the design. A LONG time ago, when synthetics first appeared, the current oils in usage were pretty crap. They had much higher volatility, (part of the reason they were crap). This meant that the rubber seals were slightly larger when in contact with these crappy oils. Now here comes along Mr synthetic. Much lower volatility, better stability etc. Now these old seals will shrink slightly, which on old worn engines meant some oil leakage.

This is a thing of the past though, seals have gotten much better since the 70s, as have the rubber used. Its just like the idea of warming an engine up completely before driving. That's a leftover from carburetors which had poor mixture control on a cold engine.

You can buy the walmart supertech synthetic diesel oil, and get at least 7k from even the larger diesels. Same goes for rotella or similar. If you are really worried just spend a bit of money for an oil analysis to see what kind of margin you have. Just once is typically enough.

The only exception is engines where the oil capacity is small, and the displacement is large (not very common).

As a note, once an oil gets heated up, even in a clean container, it gets dark in a hour or two. Still works just fine though. It only takes a few grams of soot to make oil look jet black. Doesn't change the oils properties in any significant way. In fact black oil is a good sign, it means the oils detergent package is keeping the soot suspended. Soot in the oil, is soot that's not building up in piston rings, oil passages etc.


Is it riding season yet?
I completely disagree on warm up before driving. I think it's a very important part of maintaining an engine, especially when it's cold out. Go watch a cold oil pour test and you'll see why. Even a high quality synthetic is as thick as corn syrup.

Middle of summer when it's 95f out, sure. No need to warm it up.