One of the WORST topics for discussion on forums is OIL. If you really want to know about OIL, then join Bob is the Oil Guy (link below) because that is the only thing under discussion.
You likely have a P-Series Onan engine in your tractor and while the Performer Series is not as dated as the earlier CCA and BF/BG models, it is basically the same technology. The biggest change is the introduction of electronic ignition to the P Series, which did away with the points and condenser. That is hardly what I would call a major re-design. There is no question that the Onan flat twins were a premium quality engine but the fact remains that the entire design prohibited Onan from making them comply with the new emissions standards enforced by the EPA. So in or around 2000, production of the Onan ceased. I know this to be factual because I had a telephone conversation with an engineer at the Linamar plant where Onan and Linamar engines were last built.
The older Briggs and Stratton's, Kohler's, Onan's, Tecumseh's etc were all designed to consume modest amounts of oil between oil changes. Car engines used to be the same way, which is why the service station attendant automatically opened your hood and checked the oil level of your engine every time you pulled in for gas.
In spite of all the wonderful things that modern chemistry has brought to the world of OIL, the fact remains that multi-grade oils are not the panacea for OLD STYLE engines that some people believe them to be. Multi-grade oils are full of "modifiers" that cause the viscosity to change in relation to the temperature of that oil. That is what allows an oil to behave like a 10 weight in cold weather but then behave somewhat like a 30 weight when the engine reaches operating temperature. Essentially, this is a compromise in order to provide the consumer with an all-season oil. However, a compromise is not always the best choice.
I do not claim to be an oil expert. What I do claim is that companies such as Onan, Kohler, Briggs and so forth have spent millions of dollars putting their engines through the most brutal testing possible in order to determine the BEST type of oil to use so that the consumer can get the maximum life from their product. In spite of the existence and extensive use of modern oils, NONE of these manufacturers have gone back into the Operator's Manuals for their engines and moved away from what they originally recommended.
Synthetic oils are renown for having a long life inside modern engines and the manufacturers of these oils tout that extended oil change period as justification for the higher cost of synthetics. The problem with that theory is when the uninformed try to apply it to air-cooled engines that do not have an oil filter. The crankcase is a nasty place for any oil because the old technology engines have much greater clearances then the new technology engines. Raw fuel gets past the rings when the choke is applied during cold starting and so do combustion gases that create acids and bring particulate to contaminate the oil.
In the absence of an oil filter and less than ideal crankcase venting systems, even the best oil will become contaminated to the point of causing a risk to the bearing surfaces. As these bearing surfaces become damaged, the material from them adds to the contamination and a domino effect comes into place. Synthetic oils are great but they have no more resistance to these types of contamination then real oil does. The ONLY solution to this problem is TIMELY OIL CHANGES.
Most manufacturers tell owners to change the oil every 25 hours of use MAX and more often if the engine is being used in harsh conditions. None of the engine manufacturers have swayed from that advice. Those that choose a synthetic oil in order to double the number of hours between oil changes on engines with no oil filter are not doing their engines any favour.
YOU cannot look at engine oil and determine whether it is clean or dirty. Oil will often change colour with heat. Oil filters remove particles as small as 10 microns. ONE micron is equal to one 25,000th of an inch. The human eye cannot even detect particles smaller than 25 microns and one must have excellent vision to see a particle that tiny.
The human eye cannot detect the deterioration of the modifiers added to multi-grade oils that allow them to behave at certain temperatures. The human eye cannot detect acid or fuel contamination in oil, either.
Yes, modern engines love modern oils but modern engines a built using the latest technological advances in metallurgy that allow much tighter tolerances and those tight tolerances call for oils with a 5 weight viscosity in order to assure lubrication to all parts during cold weather starts. There is no such need in old technology engines but there is definitely a need for oils with a consistent viscosity such as straight grade 30 weight oil to help cool these engines and maintain that crucial thin film of oil that keeps metal surfaces from destroying one another.
To me, it all comes down to one thing. I want to get the maximum life out of my old school engines because rebuilding these old engines can be expensive and time consuming. Therefore, it is my belief that the people who made these engines are smarter than I am and when they say I should use straight grade 30W oil in the crankcase and change that oil every 25 hours, then THAT is exactly what I am going to do.
For those who choose to listen to people that come on forums and say that their brother's uncle has used such and such an oil in his engines for the past 25 years...... be my guest. But you should take note that these same people rarely disclose the make and model of the engines nor the history, in their posts. Instead, they just make blanket statements with no real FACTS to support their claims.
This is why I suggest that people go the websites of Kohler, Onan, Briggs etc and READ what those companies suggest to be used in their engines. Now, if you think that you are smarter then all of their engineers, then by all means.... use whatever you want in the crankcase of your engines. Perhaps you will get lucky but perhaps you won't. In the end, it is your money.
Bob is the oil guy