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Heavy Oil consumption after change of oil type

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Walter349

Military
Dec 2, 2007
10
I am running a pair of 65Kw IVECO generators in a redundant configuration. We recently changed from a 5W40 Synthetic to a 15W40 mineral lube oil. Now oil consumption has risen significantly.

There is no immediate evidence from the exhaust of the oil being burned, except for some slight grey smoke on start up.

The engines are using F54 type fuel and have about 5000 hurs running time on one and about 4000 hours on the other.

Anyone have any thoughts on why this rise in consumption might be happening?
 
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The mineral oil might not be so thermally stable and is thinning out more at operating temperature or is being driven off as volatiles at operating temperature.

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Pat
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I've read alot of info people have written about having no problems going from synthetic to mineral and back again, with no problems. On the one engine I was using a semi synthetic in and then went back to straight mineral oil, I also had an increase in consumption of lube oil, it was not a diesel. I have some ideas why. But don't care to share right now, because I would have to study up on tribology again.
 
the higher viscosity oil probably leaves a thicker film on the upper portion of the cylinder wall, where it is exposed to combustion temps and evaporates... the mineral oil may also be more prone to evaporation in any case.
 
there may be two reasons or the difference. to start with: the volatility of the synthetic oil will be much lower then the conventional mineral oil. thereore the evaporation loss is higher with the conventional oil and thus the oil consumption higher. secondly: the 15W-40 oil will have a lower viscosity index then the 5W-40 oil. that means that in the region where the piston rings meet the cilinder wall the oil viscosity is lower with the conventional oil - resulting also in a higher oil consumption. a third reason might be that the viscosity index improver that is used in the conventional oil is less shear stable then the one used in the 5W-40 synthetic. that means that what originally went in as a 15W-40 oil is rapidly degrading towards a 10W-30 oil, also contributing to an elevated oil consumption.
 
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