Interesting that you brought this up, as it has been RV.net as you showed, and there was also a discussion on the Yahoo Roadtrek board. MANY folks disagree that the newer oils are not as good for the air cooled and older engines, and that is fine. MANY also insist that synthetic, regardless of API classification, are not a problem. I wholeheartedly disagree. Your older engine may last well, once engines are broken in they are much less likely to suffer a scuffing related (or any other oil related) failure, as the parts are all burnished smoother than when right out of the box. Add to that the fact that it may have been run with older, high anti-scuff oil for a long time (it stays with the surfaces a long time), and you could never know of a problem, or not.
SL was the first classification that limited phosphorus, and thus zinc (they are in the same compound), but it was not enough to cause serious problems, it appears. SM took it down further, and that is when the issues started. It first showed up (to my knowledge) on high lift, non-roller cam, hot rods, which would fail in minutes during breakin. At the time ALL the oil companies said the oils were backward compatible and would be fine on all flat tappet, not rollerized engines, water and air cooled. I know for a fact they said it, because I personally spoke with all of them, as I was in the process of building a new engine for our hotrod. Long story short is that I had two oil related failures on the newer oils when switched to them after breakin on old SG rated straight weight, and many folks I know had similar issues. We started using diesel and motorcycle oils, and the issues went away,. The oil companies continued to insist it wasn't and oil related problem
Interestingly, almost all of those same companies now sell "older engine" oil and "small engine" oil, and they are, not surprisingly, rated in the SG to SJ range, with full anti-scuff packages.
The current SN rating leave the phosphorus level the same for most oils, but I have heard that they are required to use a different compound for it, that is not as effective, so it may be worse. I have not been able to confirm anything.
Here is a link to official API specifications history document that lists the tests and requirements for current and past ratings. What you want to look for is the max phosphorus requirement that showed up on SL and later, zinc levels mirror the phosphorus. In particular look at what weights and "energy conserving" oils are covered by the limits. Everything X W 30 or lighter is covered, as is anything that is labelled with the energy conserving, but the other weights that aren't so labelled MAY have the old additive package. To confirm you have to get current specs or have an oil analysis done.
http://www.api.org/certifications/engin ... mplete.pdf
As a sidelight--While all this was going on with my, and other failures, I happened to be in charge of the test lab at a major high pressure pump manufacturer. We used, and sold, an oil specifically developed for the pumps, which had many of the same type of wear surfaces as non rollerized engines, with the only major difference being the lack of the very high temp areas seen because of combustion in the engines. We life and wear tested hundreds of pumps, and they were hard on oil. Our oil supplier supplied us with a synthetic version of our oil, just with a synthetic base stock. No change in life or energy usage. Many, Many of our customers sent in samples of oil they wanted to use so they wouldn't have to pay us for our high priced oil. Most where automotive oils, most synthetic, some with the magic additives like Militec. None of the oils came close to life and wear results of our dino oil. This included automobile oils rated SJ, and all the big names. The big difference in the additive package was that we had close to double the anit-scuff of even SJ oil. It made absolutely no difference whether it was dino or synthetic in our pumps. That said, synthetic very well could give some advantage in an air cooled engine, but only because of its high temp stability.
Bottom line for me is pretty simple. There is a long history of failures of engines of the design of our Onans when used on the newest oils. Will yours fail because of it, nobody knows, but even if you don't get the catastrophic oil related failure, are limiting the useful life? My guess is yes. Onans are full blown industrial style engines, with a minimum design life of at least 2500 hours. I have seen very few with that many hours. If you oil increases wear and takes off 20%, you still go 2000, so you won't see it.
My point in all this is that there are oils out there that remove any doubt about whether or not they are good enough, but picking up automotive, SN rated, energy conserving oil at the store will not get you that oil. Many will roll the dice and come out fine, some probably won't, but that is everybody's own choice.
Me-I had been using Mobile1 motorcycle oil in our air cooled engines, but they have reduced the anti-scuff in them, it appears. Valvoline motorcycle oil still has it.
My personal choice right now is Brad Penn oil (not their SN rated). The roundy racer guys around here swear by the stuff. Dino oil that makes their engines last longer than synthetics, including Royal Purple and Redline. It is good because it is a detergent oil that is also good for the street, and is usually cheaper than synthetics.