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Old 09-17-2020, 05:09 AM   #1
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WHY: Cummins Onan does NOT recommend Synthetic Oil

I have new 5500 Onan generator and I recently changed the oil. The Cummins Onan manual specifically states that Synthetic Oil is NOT recommended.

Does anyone know why Cummins Onan does NOT Recommend?

Note: They did not say you could NOT use Synthetic, only that it was NOT recommended.

I changed with a non synthetic 15w -40 oil that is rated for diesel & gasoline engines. I had a Onan Rep confirm the oil I chose. When I asked him why Synthetic was not recommended; he cited concerns with deposits on the cylinder walls, cylinder blocks, and emissions damage?

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Old 09-17-2020, 09:37 AM   #2
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Quote:
Originally Posted by dkoldman View Post
I have new 5500 Onan generator and I recently changed the oil. The Cummins Onan manual specifically states that Synthetic Oil is NOT recommended.

Does anyone know why Cummins Onan does NOT Recommend?

Note: They did not say you could NOT use Synthetic, only that it was NOT recommended.

I changed with a non synthetic 15w -40 oil that is rated for diesel & gasoline engines. I had a Onan Rep confirm the oil I chose. When I asked him why Synthetic was not recommended; he cited concerns with deposits on the cylinder walls, cylinder blocks, and emissions damage?
Not sure on the science but this is what Cummins says:

Can I use synthetic oil in my Onan generator?

Cummins does not recommend the use of Synthetic oil in Onan RV generators as there are no long-term benefits for the additional cost.
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Old 09-17-2020, 11:51 AM   #3
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This question has been debated infinitum on the boating forums. One Cummins guru who runs a diesel shop and sells lots of diesels says in general that there are no benefits to synthetics. But that statement depends on sticking to the manufacturers oil change recommendations. In other words during that period, regular dino oil works fine.

OTOH sophisticated truck fleet owners who do routine oil analysis and only change when the analysis says they need to can and do benefit from synthetics. Synthetics don't break down like dino oil and can last much longer.

But for the average RV owner, no benefit.

David
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Old 09-17-2020, 12:56 PM   #4
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Oil doesnít break down just get dirty. I worked at the railroad and we never changed oil. Just changed filters and add oil. We ran tests on oil every 30 days to monitor never break down but dirty. 20/50 only change was if complete engine rebuild. We changed valves pistons and other parts still no oil change just added oil. I do Chang oil in my gas rv but only ever 7000 miles. I change the oil in my generator twice a year August January. But thatís just me. Never had engine problems in any of my vehicles. Knock on wood.
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Old 09-17-2020, 01:27 PM   #5
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Originally Posted by Hugh.vines@att.net View Post
Oil doesnít break down just get dirty. I worked at the railroad and we never changed oil. Just changed filters and add oil. We ran tests on oil every 30 days to monitor never break down but dirty. 20/50 only change was if complete engine rebuild. We changed valves pistons and other parts still no oil change just added oil. I do Chang oil in my gas rv but only ever 7000 miles. I change the oil in my generator twice a year August January. But thatís just me. Never had engine problems in any of my vehicles. Knock on wood.
I changed my 5500 Onan Oil and filter for first time last weekend. I plan to do similar to what you suggest. I was going to change oil every March & September but only change the OEM Oil filter with another OEM oil filter on 1st change where I hit or got close to 150 hours? I have new Air filter but manual says 450 hours? I don't think I can wait that long to use it I would change oil in RV engine every 7500 miles.
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Old 11-07-2020, 02:57 PM   #6
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After few phones calls and lost of post of several forums, I just learned that Onan now recommends 10W30 for gas generators

https://www.cummins.com/generators/m...-rv/onamax-oil

As OP, I find this excellent!!! Make you wonder if the phone calls to Onan Support a while back had any impact?

This opens the doors to common sense and afford us many more legit & safe options.

Because of the avantage to support lower temperatures over 15-40, I will likely switch from my Chevron Delo 400 to a regular 10w - 30 oil as I use for my autos, portable generator, lawn mowers and even the RV itself. Now, (in future) I can carry 1 quart of 10w 30 on RV that can be used to top off engine or genset if needed.
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Old 11-07-2020, 03:19 PM   #7
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Quote:
Originally Posted by dkoldman View Post
After few phones calls and lost of post of several forums, I just learned that Onan now recommends 10W30 for gas generators

https://www.cummins.com/generators/m...-rv/onamax-oil

As OP, I find this excellent!!! Make you wonder if the phone calls to Onan Support a while back had any impact?

This opens the doors to common sense and afford us many more legit & safe options.

Because of the avantage to support lower temperatures over 15-40, I will likely switch from my Chevron Delo 400 to a regular 10w - 30 oil as I use for my autos, portable generator, lawn mowers and even the RV itself. Now, (in future) I can carry 1 quart of 10w 30 on RV that can be used to top off engine or genset if needed.
Your link is broke. 404 Page not found error.
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Old 11-07-2020, 03:23 PM   #8
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From experience.

The 2007 Suburban, 145,000 miles use Dino oil. Tried synthetic twice thinking longevity. Oil consumption went from quart every 2000+ miles to quart less than 1000. Sticking with conv oil.

Altima uses full syn 0-20, 120000 miles. Zero issues. Factory recommendation.

RV. Use ford semi-syn, now at 34,000 no issues never needs extra. Just finished 3800 mile trip no oil addition required.

Interesting one is race car motor. Tried syn after break in with conventional oil. Ran it a few months no net performance improvement. Much higher usage rate and more seepage. Blew head gasket and cylinder walls did not look right. Went back to conventional oil.

Synthetic makes good sense in certain applications. No added value in others. I have air cooled champion RV generator on trailer, upwards of 5000 hours on 15w40 with no issues. The Onan in RV only has 480 hrs in 4 years but gets oil change every 6 months or 150 hours.

This is all based on my direct experience.
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Old 11-07-2020, 03:25 PM   #9
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Originally Posted by EA37TS View Post
Your link is broke. 404 Page not found error.
Thank you. I fixed in original thread and providing here as well.
https://www.cummins.com/generators/m...-rv/onamax-oil
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Old 11-07-2020, 06:23 PM   #10
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I have been using the recommended 15w-40 in my diesel generator with no issues. I have changed it once and it is coming due before we go out again. I also now carry a quart with me on the road to use to top it off as needed.


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Old 11-07-2020, 10:05 PM   #11
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Synthetic oils will often not allow the engine to break-in completely, and the rings don't seem to seal as well.
At the very least: break the engine in fully with conventional oils; before switching over to synthetics..
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Old 11-08-2020, 03:26 AM   #12
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Originally Posted by Bob Denman View Post
Synthetic oils will often not allow the engine to break-in completely, and the rings don't seem to seal as well.
At the very least: break the engine in fully with conventional oils; before switching over to synthetics..
Many manufacturers are putting synthetic oil in the brand new engines, right off the assembly line. I believe Chevy was one of the first, in the Corvette.
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