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Old 09-19-2014, 08:05 PM   #1
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15w40 mineral oil vs. 5w40 synthetic in the Onan 7.5QD

I'm 50 hours away from an oil change on the Onan 7.5qd and that will mean by December, it'll be time for the change after heading out east for the fall colors.

I do think I'll be using the winterized coach for lodging over the winter during the weekends when I visit family, just like I did last winter. Would only use the propane furnace and the slide out.

I recall the Onan struggling a bit to start up in the cold using the standard 15w40.

I think going to the 5w40 for winter use will improve starts.

Any opinions about this?
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Old 09-19-2014, 08:20 PM   #2
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What does Onan recommend? Check your manual and go by that.
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Old 09-19-2014, 08:25 PM   #3
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Go by this chart.

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Old 09-19-2014, 08:29 PM   #4
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Mineral oil? yikes! I would think the struggle starting may be related to a 16 year old generator/ MH. 5w-40 motor oil will make the start-up a tad easier in colder weather.
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Old 09-19-2014, 08:39 PM   #5
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I might start on the electric side first. Check battery condition, battery terminals, starter connection, etc. Starter may just be tired.

Not sure that the lighter weight oil will substantially ease the start but worth a try. It's generally good to stay in the manufacturer recommendations.
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Old 09-19-2014, 08:49 PM   #6
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I'd go with the 5W40 synth for several reasons.
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Old 09-19-2014, 11:19 PM   #7
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I know first hand that 5W-40 synthetic oil does not work in my 1999 Dodge pickup with the 5.9 liter Cummins engine. Engine developed excessive blow-by and my mechanic said you cannot use the 5W-40 as a substitute for 15W-40 in older diesels.
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Old 09-20-2014, 06:59 AM   #8
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Use the chart posted by pasdad1 above.

If you continue having hard start issues then you should have the glow plugs and the glow circuit itself inspected and tested. The QD series should start without hesitation in the worst of winter conditions.
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Old 09-20-2014, 09:06 AM   #9
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I have no desire to run the syn oil in my 5.9.

Although my Onan did start every time in temps around 10-20 F, it was a bit labored and I always cringed when starting it, hoping the parts could get oil quick enough to not run the risk of an increased wear of parts due to the very cold syruppy oil.

I wish there were a 15w40 syn oil available if only for the properties of syn oil tending to remain on parts and not drain off much like mineral oil does.

Case and point where I ran a 93 Ford Aerostar almost completely out of the Mobil 1 that was in it on a 140 mile trip from Ohio to Michigan due to a common oil pan pinhole leak from rust. The engine already had 160k miles and never burned any oil, previously. After putting four quarts of Mobile 1 back into the 5 quart capacity engine, oil consumption was unchanged until I sold it a couple years later. My mechanic told me it was the Mobile 1 that saved the engine and that I would have very different results had I been using mineral.

I think I may still use the syn and closely monitor oil consumption as a precaution.

Oh, the API standards are met with the syn...
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Old 09-20-2014, 09:11 AM   #10
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Bolt- Seems there are a few...List: Full Synthetic - 15W-40 | O'Reilly Auto Parts
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Old 09-20-2014, 09:34 AM   #11
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Bolt... I Agree with post #10 above. The best oil IMO for your generator is Royal Purple ( HPS 10w-40 )Royal Purple HPS HPS10-40 - High Performance Street Motor Oil | O'Reilly Auto Parts
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Old 09-20-2014, 10:10 AM   #12
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15w40 mineral oil vs. 5w40 synthetic in the Onan 7.5QD

What are the temps expected with the next oil change interval of the generator?

Notice the 15W-40 should NOT be used in temps lower than 10

Are you expecting temps down to zero? Then I would go with Mobil 1 10W-30 synthetic or 10W-30 conventional
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Old 09-30-2014, 04:49 PM   #13
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OK, so I am jumping in to try and learn about oil for my Gas Onan.. Mine says 30W or 15-40..I like the idea of the 15-40 but it is harder to find.. I need to understand the SJ,SH, SG designation. Is a SM designation Ok to use.. I think someone said that SM would be just a later and maybe better designation than SG.. Mine is 10 years old.
All help appreciated,, Don
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Old 09-30-2014, 04:58 PM   #14
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Read your owners manual. It will tell you what oil to use for the expected temperature range you plan most of your operation to be at.
I use only synthetic and I change the oil every 50 to 75 hours or earlier if I am making a big change in temp range I plan to use the genset in. I do this myself as oil changes are cheap and easy
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