Here's my $.02. worth.
All our mower and air cooled engines here are at least 10 years old, (some are over 20) and I wasn't aware that apparently the newer ones manuals are now recommending using a multigrade oil, which is interesting, because that's all I've ever used-once a machine was past warranty (or after the first oil drain). Everything (4-stroke/4-cycle) that we have manuals on hand for reccomends using SAE 30, although some said 10W30 or 40 use was permissable, but to check the oil level more often for increased oil consumption if using those oils. In high hour usage, this seems to bear out as true-which I attribute to the increased volitization of the lighter weight oil from an air-cooled engine's high heat levels in normal operration over that of a typical water cooled-engine (for which multigrade oils were originally designed). In fact, I've even taken to using oil up to 20W50 synthetic in the garden tillers as they get such heavy load usage here in our mostly clay and rock soil that they tend to run on the hot side.
I previously used 10W40 "semi synthetic" by Castrol, but switched to their same designation version of 15W40 "truck" and then to their as-labeled "diesel" oil a few years ago for most things with no oil filter and especially for those that I operate only in the warmer months. I figure that all other things being equal (meaning similar additive packages), that since a diesel engine typically puts a much higher pressure load on it's rod and crank bearings, an oil designed for use in one should hold up longer/better to pressure breakdown in a non-diesel engine too, and that it might actually resist temperature degradation better as well. Another side benefit of a diesel engine oil is that most good ones are designed to carry and still function well while carrying higher soot (think of dirt) loads than a typical "car" engine is designed to carry-which ANY engine that has no oil filter on-board should theoretically also benefit from. (If someone has better information or info to the contrary, I'd be happy to be better-educated on this aspect.)
BEFORE the warranty period expired I'd already started using 10W40 semi synthetic Castrol for the gas powered 4 cycle 1997 garden tractor's engine, and when it became more available in our area (east Tennessee) I switched to their slightly heavier above-mentioned 15W40 version synthetic (or semi synthetic) or a mix of weights of semi sythetic and full synthetic "truck" or diesel oil. That machine I use year-round, (mostly for pulling a cart in colder weather) and aside from the reduced available battery amperage in Winter making for a slower cranking speed it has no trouble starting in colder weather with this oil, which is actually theoretically a "lower" viscosity under cold conditions than a straight 30W would be.
The tractor's engine has well over 2000 hours on it by now. I pull the plugs to check them from time to time and regap them as necessary, but they tend to stay very clean. I think that I'm on the 2nd set of plugs now-but then I ONLY change spark plugs when their electrodes actually get too thin for my taste if they're working properly and are in good shape physically otherwise. I've changed the head gaskets once so far (which was shortly after the warranty period expired, to de-carbon the heads. As it turned out there was minimal build up then. Synthetic oil typically does form carbon deposits much more slowly than a conventional motor oil will in a properly operating engine, but it's probably time to check the heads again. This engine has had a long-term slow oil leak, so I can't say how much oil it's actually losing from internal consumption vs leakage.
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P.S. IF I recall correctly, I believe that the camshaft problems GM was having years back that has been referenced in this thread was actually later proven to be from improperly hardened camshaft lobes and that it was brought out in court that the "wrong oil" excuse was a calculated corporate cost/benefit analysis attempt to escape eating millions of dollars of legitimate warranty coverage-one that was fought by a class action suit after too many customers got screwed and one of those with good records got very mad and started to raise a stink. This would be only one in a long string of times that an auto company has done this cost/benefit of warranty vs litigation calculation sort of thing. But I could be wrong about this particular one.