As interesting as many BITOG discussions are, this is an example of what can rub me the wrong way. Everyone's entitled to their opinion, and here are some things on my mind.
First, in the PCMO and other similar forums when a poster asks what oil they should use the first responses are "what does the manual say?". Here on the small engine forum, few seem to care what the manual says.
All of my OPE manuals state 10W30 for all temps, 30W above 10C (or something like that). They don't list any optional 40W or 50W ranges for ANY temperatures. A *lot* of people seem to be in the same boat with their brand of engine (mine are Honda).
Second, don't you guys think the folks who built the engine know the typical operating temperatures? Don't you think they know what viscosity SAE30 is expected to be at those temperatures? Why are you convinced that we're onto something that professionals in the field haven't figured out yet? I've seen dozens of threads where folks use the logic above "if a 30W is x cSt at 100C we have to use a 40W which will be the same cSt at the higher temperature". Why? Maybe the engine is designed for (x-2) cSt at full temperature.
I'd consider the higher-than-water-cooled temperature a possible justification to use synthetics or synthetic blend so they don't cook and leave deposits but not as a universal need to switch grades. If the engineers thought that the high heat really needed a 40W, wouldn't they put 10W40 or 15W40 in the manual? ...even if only as an option for high ambient temps? It's not like it's hard oil to find and they had to dumb it down like some consumer grade compressor manufacturers specing 5/10W30 PCMO in a "pinch" instead of ISO100 (example).
Third, the engineers who design these engines must know their temperatures and where the hot spots are to specify proper machining tolerances and metallurgy. We've discussed before how single piston OPE engines have cylinders/pistons that go elliptical when cool and have sloppy tolerances until warmed up. Doesn't that mean that as they really get hot they continue to get tighter? If I understand correctly, as gaps decrease, thicker oils may no longer be suitable protection. Is this the time you want heavier oil than the manufacturer recommends in there instead of something you know will maintain a film on the cylinder wall instead of getting wiped clean away? The leading edge element in the PCMO forum is "going thin" and debunking that "thick is always better" over and over again but some here are hung up on putting dino 15W40 in their snowblowers. When the clearances are large at cool starting temps, a single-grade 30W will also be quite thick to allow rings to seal etc... That seems to work pretty well with the needs of the design. Perhaps it's in the design that 30W oils get appropriately thin when in high heat conditions to splash easier and stay where they are supposed to be.
High TBN in itself is an admirable goal. I know that most people (myself included) won't pay for a UOA for their lawnmower to see if ultra-high TBNs are truly required at proper OCIs. There is already a good selection of high TBN oils without going to 15W40 diesel oil. (Amsoil ASE, M1 MX4T I think, various dino 30 weights)
OK, I feel better now
In the end, one thing I've seen mentioned here is that the engine always seems to outlast the rest of the equipment it's attached to, and there's probably nothing you can do to kill it. I, like many, like to do the best I can for my gear especially when it's a major investment so I/we obsess over the right fluids. Carry on!
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