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Old 03-10-2022, 06:54 AM   #1
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Best oil to use in an Onan Generator

What is the best oil to use in an Onan RV QG 4000 gas generator. Straight weight 30 SAE, Multiviscosity 15w40 and is it ok to use a full syntheticc in the genset? Thanks
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Old 03-10-2022, 06:59 AM   #2
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Go to Amazon or a local dealer and get Onan oil and air filters. Keep your receipts in case you have any issues.
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Old 03-10-2022, 07:02 AM   #3
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What is the best oil to use in an Onan RV QG 4000 gas generator. Straight weight 30 SAE, Multiviscosity 15w40 and is it ok to use a full syntheticc in the genset? Thanks
Do you have your manual? I'd refer to that and see what it recommends.
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Old 03-10-2022, 07:04 AM   #4
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What is the best oil to use in an Onan RV QG 4000 gas generator. Straight weight 30 SAE, Multiviscosity 15w40 and is it ok to use a full syntheticc in the genset? Thanks
I run straight 30w but it's hard to find these days... 15w40 works too. I don't run synthetic and change it every 100 hours or year depending. There should be some info listed on your genset itself.
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Old 03-10-2022, 07:05 AM   #5
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Check your Onan manual.

Ours (5.5JGJAB) says 30 wt. for operations 32 degrees F and above, and there are 3 other temp conditions scenarios described there as well.

As for type of oil, they say Class SJ, SH, or SG.
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Old 03-10-2022, 07:28 AM   #6
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I run 15W40 synthetic (Delvac Extreme) in my genny. Why? Synthetic is much better oil than conventional oil according to every engine builder or oil engineer that I've ever talked to, and I've had conversations with a few. That and it's the same oil I use in my engine so I only have to carry one jug of oil. The reason that I run multi-viscosity is that it's thinner when you need it to be thin (cold starts) and thicker when you need it to be thick (hot weather). Although straight 30 weight is recommended for gas generators from 32 deg to 100 deg F, 15w40 is approved from 10 deg to 100 deg F per my generator manual which seems to cover all of their generators. If I were you I would go with straight 30 weight unless I thought I might be in colder weather then I would go with 15W40. In either case I would go with synthetic oil. It doesn't make any sense to me to risk a several thousand dollar generator to save $10 once a year. Just my two cents. Good luck.
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Old 03-10-2022, 07:32 AM   #7
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What is the best oil to use in an Onan RV QG 4000 gas generator. Straight weight 30 SAE, Multiviscosity 15w40 and is it ok to use a full syntheticc in the genset? Thanks
I have the same Genset. The manual says 30wt. Like someone mentioned, straight 30wt is becoming hard to find. 10w30 would be fine. Synthetic is fine, your choice. The class designation of S-, today I believe it's up to SN. It changes with time, so today's SN is backwards compatible with older S_ designations. I also believe my manual said to change oil every 50 hours. I could be wrong. I will go out to the coach later and look at the manual again.
Get yourself a good brand of oil, 10w30, make sure it's API designation starts with an N, NOT C, C is for diesel, and change your oil. No oil filter on mine, yours most likely doesn't either.

Do you have the Onan MANUAL??
There you go. Take care.
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Old 03-10-2022, 07:50 AM   #8
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why is there so much concern about get oil. just use Onan recommended Onan oil 15w40 . covers your warrantee. go to Amazon and order oil. case closed
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Old 03-10-2022, 08:34 AM   #9
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why is there so much concern about get oil. just use Onan recommended Onan oil 15w40 . covers your warrantee. go to Amazon and order oil. case closed
Because the Onan brand oil is not better or sometimes even equal to the many other oil brands out there. There are better oil choices.
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Old 03-10-2022, 08:46 AM   #10
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possible true but if you are under warrantee better use their oil until you are out of warrantee. any excuse to NOT WARRANTEE YOUR GEN IS ALWAYS A POSSIBILITY. I DO NOT LIKE THE RISK.
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Old 03-10-2022, 08:52 AM   #11
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I use the same oil in the engine and generator. Meets Onan's requirements. I use synthetic. I've always felt synthetics perform better in my air cooled motorcycles, so I figured why not use it in my air cooled generator. The reality is, synthetic or conventional is a personal choice. The key to long engine life is following the recommended oil change intervals using approved fluids and filters. Beyond that it's whatever makes you feel better.
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Old 03-10-2022, 08:54 AM   #12
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Because the Onan brand oil is not better or sometimes even equal to the many other oil brands out there. There are better oil choices.
Not to mention, it's higher in price.

I'll have to look next time I go to the oil store (Wallyworld) to see if they still have any 30W oil. That's what my manual says is best for our normally above freezing weather and is what I've always used in my 19 year old Onan.
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Old 03-10-2022, 09:05 AM   #13
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possible true but if you are under warrantee better use their oil until you are out of warrantee. any excuse to NOT WARRANTEE YOUR GEN IS ALWAYS A POSSIBILITY. I DO NOT LIKE THE RISK.
The manual for my 4000 states to use oil that meets the API standard SJ and to use the correct viscosity for the temperatures I'm running the genset in. Surprisingly, it doesn't even mention the Onan oil.

Performing required maintenance, using oil with the correct specifications, and keeping your records and receipts does the trick for warranty coverage.
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Old 03-10-2022, 09:11 AM   #14
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What is the best oil to use in an Onan RV QG 4000 gas generator. Straight weight 30 SAE, Multiviscosity 15w40 and is it ok to use a full syntheticc in the genset? Thanks
Here is a link to all the Onan generators, it may provide the answer. Hope it helps you.

https://www.cummins.com/rv-generator-manuals
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