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Old 07-13-2021, 08:04 AM   #1
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What oil for 4KY Onan?

My 1998 Onan gas-powered generator ops manual calls for SAE 30 (SG or SH) motor oil. My search for straight 30 weight oil is making me think it's not available except in "house" brands. Onan says 30 weight "is preferable when temps are consistently above freezing." I'm somewhat reluctant to go with a house brand.

Alternatively, Onan says 15W-40 (OnaMax) for 10 to 100 degrees Fahrenheit. Also having trouble finding that, other than by mail.

Found Castrol 10W-40 SG Motorcycle Oil. Seems like motorcycle engines would come closer to the operating environment of an RV generator (air-cooled, constant RPMs, etc.).

The generator only has 193 hours but I have no way of knowing when or if the original owner (I'm number 2) changed the oil or if it's original.

Leaving very soon on a 2k mile trip and suspect I'll be using the house air conditioner more often than not given the temps all over the country. As good as the ginny has been, I really would like to start out with new oil but can't decide.

Thoughts, especially from first-hand experience?

TIA
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Old 07-13-2021, 08:07 AM   #2
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I order the Onan 15w40 from Amazon for my 5500. My brother has always used straight 30 weight, and has over 1100 hours on his genny.
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Old 07-13-2021, 09:22 AM   #3
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RESOLVED but still interested in opinions.
Elwood58, was able to find 15W40 at Cummins in Tulsa. Was able to get two quarts put on hold until I drive by there in a couple of days. They said the 15W40 was their most popular, BTW, and only had five left in stock. Thanks for your input.
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Old 07-15-2021, 02:15 PM   #4
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GuyG: I've found at Walmart all different weights on the oils. Pick your preference. When it comes to oil, for example, my little Prius C has its oil changed every 5000 miles or sooner. I always viewed that as cheap insurance. And I change it whether it needs it or not!! And I'd bet that in the long, long run you are saving money that way. Try it. If you think it hasn't been changed or just not sure, change the damn oil ! You're talking about a few bucks here. Happy travels.
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Old 07-15-2021, 08:04 PM   #5
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SAE 30 is commonly available from most major brands though it will generally be labeled the newer SN grade, I usually use 15W40 for my Onan 4000 KY as I have it on hand for various diesel engines.
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Old 07-15-2021, 10:20 PM   #6
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Quote:
Originally Posted by GuyG View Post
My 1998 Onan gas-powered generator ops manual calls for SAE 30 (SG or SH) motor oil. My search for straight 30 weight oil is making me think it's not available except in "house" brands. Onan says 30 weight "is preferable when temps are consistently above freezing." I'm somewhat reluctant to go with a house brand.

Alternatively, Onan says 15W-40 (OnaMax) for 10 to 100 degrees Fahrenheit. Also having trouble finding that, other than by mail.

Found Castrol 10W-40 SG Motorcycle Oil. Seems like motorcycle engines would come closer to the operating environment of an RV generator (air-cooled, constant RPMs, etc.).

The generator only has 193 hours but I have no way of knowing when or if the original owner (I'm number 2) changed the oil or if it's original.

Leaving very soon on a 2k mile trip and suspect I'll be using the house air conditioner more often than not given the temps all over the country. As good as the ginny has been, I really would like to start out with new oil but can't decide.

Thoughts, especially from first-hand experience?

TIA
I use Rotella T6 15w40 in mine. It runs well and burns very little. I have 800 hours on my Onan 5500.

Make sure you follow Cummins other maintenance specs such as air and fuel filter changes, spark plugs, valve adjustment etc. Also make sure you exercise it under load for 2 hours monthly when not in use.
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Old 08-02-2021, 03:32 AM   #7
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A great oil for an air-cooled generator is Valvoline VR-1 Racing (conventional) 10w-30. It is widely used in the ACVW/Porsche community.
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