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Old 07-13-2017, 08:48 AM   #15
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Others have already said it but 15w-40 and 5w-40 are the same thing at normal operating temperature.

You have something else going on. I'd look and make sure one of the filters isn't leaking oil and causing you to be low on oil. Check the dipstick too and make sure they didn't underfill it.

I would always use exactly what the book says if you intend to ever get warranty work on the engine. If not, they'll blame the "wrong" oil for any problem you have.
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Old 07-13-2017, 04:25 PM   #16
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Is the Cummins like a CAT, using oil pressure to run the fuel system?
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Old 07-14-2017, 08:14 AM   #17
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the issue with a low viscosity oil is you don't get the payoff that exists in gas applications. Gas motors have tighter clearances. The lower weight oil helps with cold start but requires more viscosity improvers (and use different base stocks) that are most subject to breakdown as they age. All you end up with is thinner oil at the end of the service life. Due to the size of diesel bearings, they can live with the short period of low oil pressure (and comparatively infrequent ) at cold start.
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Old 07-14-2017, 09:15 AM   #18
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Replace the filter(s), make sure everything is tightened, put the right oil in, and drive it to check operation. I doubt anything else is going on.
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Old 07-14-2017, 09:28 AM   #19
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Hi... here is my update on the engine oil mashup: Bellringer RV Service in Waco, TX agreed that a mistake had been made and they drained the generator and engine oil and replaced with Delo Isosyn 15w-40 on their dime. All seems well now. I've done research on the Cummins 6.8.3 engine. It is perfectly normal for this engine to idle, when fully at operating temperature, at oil pressure as low as 5-10 pounds. At say 1600 RPM, under load, oil pressure will be about 55-65 pounds. Cummins informed me definitively that they do NOT recommend 5w-40 oil for their engines, or Onan/Cummins generators. After the second oil change, I'm getting about 17 pounds pressure at idle, at operating temperature (185 degrees F). So all seems OK now. BTW, I've learned that the 6.8.3 Cummins is a low oil pressure engine. Apparently, the oil pump is designed to only pump at low pressure. Some owners report that when warmed up, they get only 5lbs or less pressure. This is a known issue with this engine but they run and run and run for many many thousands of miles.
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