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Old 03-08-2015, 07:27 PM   #1
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Oil for Onan gas generator?

Our Greyhawk has a built in 4000w Onan gas generator. The recommended oil viscosity table from the Onan manual is attached. The 10w-40 temp range tops out at 80 degrees, but the 15w-40 viscosity is good up to 100 degrees. I thought the "40" designation applied to the max temp for any oil. So why would two "40" weight oils have differing upper limits?

Given the wide range of temps we will encounter, I would prefer to run a multi-viscosity oil. However, in the local auto parts store the only 15w-40 oils were for diesel engines, and my Onan is gas.

What viscosity oil you run in your gas generator and is it one of the weights recommended by your manufacturer? Does anyone run a diesel oil in their gas generator?

Thanks.
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Old 03-08-2015, 07:53 PM   #2
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They are more concerned with the cold temp so you get adequate lubrication on start up. They do prefer you to use 30 weight unless you camp all year in heat and cold. With that said Rotella would work fine as well.
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Old 03-08-2015, 09:22 PM   #3
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I always use Mobile 1...same as I put in my Corvette's. I've never had a problem.
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Old 03-09-2015, 06:31 AM   #4
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Rotella here. And I only have to use one oil type since I have Rotella for my Duramax.
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Old 03-09-2015, 07:00 AM   #5
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If its still under the Onan warranty, you need to use the Onamax replacement parts/oil. If problems arise, Onan may/can refuse a claim due to "use of non approved items".
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Old 03-09-2015, 08:12 AM   #6
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I have bought Onan oil for mine two times now. The first time I bought at an RV parts place for $9.50 a quart. The second time I went to the Cummins/Onan place and the parts guys says to me "you know you can use any brand oil, right?" At that point I was there, so I bought 6 quarts at $7.50 a quart. Next time I need oil I will be running non-Onan brand.
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Old 03-09-2015, 08:39 AM   #7
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There is no "simple" answer but the "w" refers to the winter rating. The lower the number, the better it flows in colder temps. The other number is at operating temperature and the higher the number, the more heat it will take. Here is the technical explanation.
Motor Oil Viscosity Grades Explained in Layman's Terms
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Old 03-30-2015, 01:13 AM   #8
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thanks NC Roamer for asking the question - timely for me too.
I have a 5500w in a 2010 Recon I just bought used. Getting it ready to hit the road.
Has right at 80 hrs on the genset. Going to change the oil and filters and service before we leave. Don't care much re the warranty, probably not covered anymore anyway. But very interested in the Synthetic oil discussion. I see Rotella mentions several times, and since I'll be servicing the TV before we leave as well (F450) I'm thinking the same oil for both? Edatlanta - I see you run a diesel, are you happy with the synthetic?
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Old 03-30-2015, 04:56 AM   #9
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I use Rotella T in my Onan and Duramax, but not the synthetic. I'm not a big fan of synthetic oils.
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Old 04-01-2015, 06:37 AM   #10
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Please remember to use some sort of fuel additive in your gasoline, and start up the genny for a few minutes each month. Bad/old gas is the #1 problem we see at the shop. $7 bottle of sea foam in the tank when you plan to run the generator works wonders. Just got done replacing a carb on a microlite generator.

just my $.02
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