Oil for Onan gas generator?

[NC Roamer]

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.

[Grumpy]

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.

[Ustabslim]

I always use Mobile 1...same as I put in my Corvette's. I've never had a problem.

[edatlanta]

Rotella here. And I only have to use one oil type since I have Rotella for my Duramax.

[John LLRV]

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".

[TX_Arvee]

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.

[Roadwarrior56]

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

[SSBN608]

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?

[edatlanta]

I use Rotella T in my Onan and Duramax, but not the synthetic. I'm not a big fan of synthetic oils.

[John LLRV]

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

[SSBN608]

Sea Foam?

[Ela1948]

Sea foam is a fuel additive that has been around for years. Walmart or Amazon carry it. Have used it on outboard motors where fuel may not get cycled as often as desireable. It is 100% petroleum product.

[SSBN608]

huh. I've never seen it. great heads up thanks.
I need to service my genset and I was worried about old gas.
I'll pick some up, thanks.

[reitrof]

I use the same oil as in the engine Mobil 1 0w30.

The genny needs to be run on a regular basis. According to Onan, 2 hours under a load each month if not in use.

I would careful about pouring in additives since in the Greyhawk, the genny uses the same tank as the engine. Also if sitting for awhile, more than 2 weeks, use a fuel stabilizer for both the engine and genny.

[SSBN608]

I'm pulling a TH with an onboard Onan 5500, and the TH has 2 x 27gal gas tanks, one for the genny and one for the toys.

The TV is a 2013 F450. I normally run the Delo 400 whick works great year round in AK. But going way south for the summer and struggling with a good oil for the Hot weather and 18k load.