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I have a 1983 JD 318 with the original Onan B43G engine. Since it's getting harder to find conventional motor oil, has anyone tried/switched to a synthetic blend motor oil? Any reason to think this may harm the engine or possibly be better? Any comments would be appreciated.
 

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Steve,

Oils in general and synthetics in particular can elicit quite a few widely varying opinions. But as to function, using an appropriate weight synthetic or blended oil is going to be OK in your Onan... Blends tend to be a little less costly that the full synthetic, of course.

Cost is what prompted this statement from Cummins regarding the Onan engines:

Cummins does not recommend the use of Synthetic oil in Onan RV generators as there are no long-term benefits for the additional cost.

Chuck
 

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From what I recently learned, you can use multi-viscosity oils (dino or synthetic) in anything that has an oil pump and filter. A motor with no filter should use straight weight oil, which is now getting harder to find because of the lack of market.
 

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Elaborate please.
 

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This is probably the biggest internet debate in our area of interest. I’m no engineer and I do what works for me so if anyone dislikes my statement so be it, save the flames for someone else. The engine was designed to run on conventional oil. You want to drop a valve seat, go ahead and run synthetic. You want excessive smoke, go ahead and try it. These old flat heads use oil and love to puff on startup, throttle up and governor up. Synthetic will exacerbate the symptoms. I’ve never thrown a rod, cam or a seat and I’m going to stay with my conventional oil. You will void your warranty from Durkan’s Deere farm if you try any of that crap in one of my machines! I’m out 🤣
 

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So are you talking about engines that have oil pumps/filters or not? Or is this just about dino-vs-synthetic? Synthetics are a blend of dino + man-made additives right?
 

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Like I said above "can elicit quite a few widely varying opinions." Strong ones too...

Tom -- blends are a mix of dino and synthetic, but as I understand it synthetics can be made that contain no dino oils...

When searching this topic there is quite a lot of information, but it is often completely submerged by the more numerous opinions on the matter. As to composition, here is a good summary:
Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. Major classes of synthetics are mostly either of sevaral major types:​
Poly-alpha-olefin (poly-α-olefin, PAO) is a non-polar polymer made by polymerizing an alpha-olefin. They are designated at API Group IV and are a 100% synthetic chemical compound.
Polyalkylene Glycol (PAG) Synthetic Oil -- PAGs are not compatible with mineral oils, most seals, paints, varnishes, and natural rubber, Buna-N, and most regular seals are incompatible with PAG oils, especially seals coated in mineral grease. PAG oils can cause seals to shrink or swell, thus causing severe leakage or seizure of the seal.​

Semi-synthetic oils (also called "synthetic blends") are a mixture of mineral oil and synthetic oil, which are engineered to have many of the benefits of full synthetic oil without the cost.​
Read the full details here: https://en.wikipedia.org/wiki/Synthetic_oil
Clearly the PAO oils which are compatible with mineral oils are used for the 'blends' and can be used in most engines. Whether you choose to do so is mostly a personal preference more than science...

Chuck
 

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FWIW:
Have used nothing but Castrol 30W oil in our 1980 111's engine for over 35 years.
None at wally world, or other places when I was looking for some 2-3 months ago.
Good ol' local NAPA store had just what I wanted - on the shelf.
 

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I found straight weight 30 recently at Napa and stocked up on it, it was actually quite reasonably priced
 

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On the other hand-- I run synthetic in everything with no ill effects. There's no major benefit to running a synthetic oil, you typically can't extend your service intervals on these air cooled engines. There's also no real drawbacks as far as I'm concerned. But at then end of the day, I can usually get full synthetic oil cheaper than I can get conventional (kirkland brand at costco, usually goes on sale for $27 for 10 qts).

Now there may be something to using a conventional oil during break in for better ring seating, but that's a different argument.

And for what its worth-- I am an engineer, but I'm certainly no tribologist 😄
 

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Whenever I change the oil on something, I drain the plastic container through a funnel into a gallon jug. This jug eventually has 5-20, 5-30, 10-30, 10-40 15-40 and 30 wt. in synthetic and non-synthetic all mixed together. That's what goes in the lawn mower, tiller, etc. Been doing that since the 80's.
 

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As an engineer who HAS studied tribology, @chuckv and @Brandon.S nailed it . If you change your oil with any regularity, odds are you'll be OK. Oil changes are mainly to swap out oil that's contaminated with diluted fuel and water. I'm not willing to run "extended" oil changes, on the basis that it isn't THAT expensive to put 3qts in my 332. I still use synthetic, everything in the fleet (including cars) runs the same stuff at my house.

There are certainly chemicals in the recipes that may or may not be compatible with some seals and such. PAG oil is used in AC systems for the compressor. I don't know off hand any other mainstream uses for it personally.

Most "conventional" oils now are "blends" actually. I wouldn't stress too bad about finding conventional, it's unlikely to find one that is 100% conventional these days anyway. You've probably had 25% synthetic run in your machine by now, just haven't known it.

I'd personally go more out of my way to get E0 gas. Different conversation though

-Aaron-

Sent from my SM-G781V using Tapatalk
 

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The only time we run conventional is the odd occasion we get a couple bottles of straight SAE30 Dino out of a Gator service kit and gets dumped into whatever just needs its oil changed. Otherwise it’s all synthetic multi weight
 
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