there are two different specifications in the onan gas or lp manual. start there. there is probably a couple of others for the diesel variety. the difference is the viscosity of the oil based upon the prevailing ambient temperature. of course the manual says to use onan branded oil. many people have said that the onan oil is made by valvolin, but i've never seen anybody state that the onan oil is exactly the same as the valvolin oil. but let's be honest, do you think they spec the onan oil for performance reasons or for profit reasons?
onan will also say not to use synthetic oil but it you push them a bit on this they really don't have a reason. could it be that they haven't licensed a onan brand synthetic?
you'll get many different answers as to what oil to use.
most probably using any quality oil of the recommended viscosity and changing it and the filter on schedule is far more important than what brand you are actually using.
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2015 cardinal model 3825fl
2015 dodge ram 3500 dually
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