A couple of old manuals I have around the house state SAE 30. Prior to engine rebuilds I have used non detergent SAE 30 from the local Tractor Supply just to get engines started and to putt around the block to establish that things work.
The oil of choice around these parts seems to be Shell Rotella T4 15W40, although marketed as a diesel oil a lot of local guys (myself included) use it for flat tappet stuff because of the higher ZDDP content. Last I heard it was running somewhere in the 1200 ppm zinc and 1100 ppm phosphorus range. I have been running this in my truck since a basic rebuild, in my 235 since its full rebuild, as well as my brothers classic Fiat with no ill effects, however I have never sent any samples off for analysis so my experience is purely anecdotal.
It's API rating is SM for gasoline (recommended for 2010 and older) and CK-4 for diesel; which taken from the API's website: "API Service Category CK-4 describes oils for use in high-speed four-stroke cycle diesel engines designed to meet 2017 model year on-highway and Tier 4 non-road exhaust emission standards as well as for previous model year diesel engines. These oils are formulated for use in all applications with diesel fuels ranging in sulfur content up to 500 ppm (0.05% by weight). However, the use of these oils with greater than 15 ppm (0.0015% by weight) sulfur fuel may impact exhaust after treatment system durability and/or oil drain interval. These oils are especially effective at sustaining emission control system durability where particulate filters and other advanced after treatment systems are used. API CK-4 oils are designed to provide enhanced protection against oil oxidation, viscosity loss due to shear, and oil aeration as well as protection against catalyst poisoning, particulate filter blocking, engine wear, piston deposits, degradation of low- and high-temperature properties, and soot-related viscosity increase. API CK-4 oils exceed the performance criteria of API CJ-4, CI-4 with CI-4 PLUS, CI-4, and CH-4 and can effectively lubricate engines calling for those API Service Categories. When using CK-4 oil with higher than 15 ppm sulfur fuel, consult the engine manufacturer for service interval recommendations."
The only two issues I have come across are that if for some reason you are running catalytic converters (which none of mine are), it has a tendency to gum these up due to the higher zinc and phosphorus; and second, be prepared for slower cranking in extreme cold.
To each their own, in the end it is most likely all snake-oil anyways. Justhorsenaround hit the nail on the head with "Good oil and regular maintenance will keep you going a long time."