Chevy and all the other good folks in here to date...

I'm surprised that no one has yet mentioned the effect of the basic fuel liquid in diluting the oil fraction...

Petroleum oils dissolve in gasoline (likely also in kerosene), vegetable oils in methanol. (Generally, and oversimplified...)

As to maintaining an oil film once established: the basic fuel component is solvent for the oil, and can as easily flush it off the wearing surfaces as help bring it to them. That, and the added area of plain, full-contact bearings make a good first point in why manufacturers often - if not always - recommend more oil% for plain bearing engines. The second point mentioned in here is that diesels generally burn much less fuel for a given time. With less fuel passing through the engine per microsecond, each increment's fuel plausibly needs a bit more oil to maintain adequate lubrication. Our glow engines slosh bunches of fuel through; our diesels do not.

Of the oils: the synthetics available in the 1970's, when Dave Gierke did his classic engine test articles for Flying Models mag, burned off at combustion chamber temperatures indirectly measurable on glow engines run leaned out (ST 40, possibly a G21-40?). They burned off at 485°-490°F, IIR. Castor didn't, it held its mud to 500°-510°F, IIR. Measured head temps were between those two ranges... Implication? Castor is heated by conditions as it goes through an engine, and by staying intact, unburnt, carries the heat it absorbed out the exhaust. That is a cooling advantage. Synthetics, unless they have raised the flash point 20°-30°F since Dave G's tests, burn away - adding heat inside the engine instead of taking it with them as intact, hot droplets, out the exhaust port.

(The lubrication effects of synthetics burning above the piston on the power stroke seem to be insignificant. Nothing needing lube is in there, and the generous transfer porting in today's schneurle layout engines douses the upper sleeve with ample fresh, cooler fuel/air mix to lube the piston on the way up. A cyl/piston pair which, btw, is most often cut to fit best at operating temperatures, thus is a bit less dependent on the oil for survival...)

Empirically, glow fuels DO cool the lower crankcase very effectively as the methanol - at least - vaporizes out the spraybar/throttle nozzle. You can check this on the test stand by sliding a finger under the case, below the venturii/carb as it runs. It is cooler, if not actually cold to the touch, unless the engine is run extremely hot. Even then it is cooler than the outer case from the base of the exhaust stack up.

Diesels, and the few spark engines, I've run do not have the 'heat of vaporization' advantage of methanol-based fuels, and soak hotter throughout. (I've noticed that diesels do not get as hot up top as glows, but soak to a more uniform temp, overall.) Ether does chill on vaporization, but there's usually about half as much by percentage as there is methanol in glow fuels. Kerosene does not chill much on vaporization, nor does gasoline.

We have the ability, with modern metals and manufacturing techniques, to produce inexpensive engines that require a very low oil% in pre-blended fuels. Traditional model engines, and even our most modern-technology engines, usually require much more oil than those (weed-whacker, lawnmower, etc.) engines, partly to absorb and eject heat. Some few modern model engines are fitted and metal-matched so that they DO NOT run well on castor, or castor+synthetic fuels. Part of that may be the tendency of castor fuels to deposit a varnish-like coating on the hot wear surfaces they are exposed to. That is great for 'porous' cast-iron pistons in steel sleeves, but not so nice for chromed, or nickeled, softer metal sleeves running high silicon aluminum alloy pistons. ANY varnish depth ruins the designed and intentionally developed operating temperature fits.

As to diesel manufacturers recommending lower oil% for racing purposes - the general presumption seems to be that:
1) a good break-in comes before competition use, so that heat cycling, fit burnishing, and any wear-tempering processes are well established.
2) Control-line racing, at least, is NOT at an engine's flat-out ultimate output conditions. It is eased a bit so that pitting restarts are dependable. Overheated engines do not restart well... And, laps per tank fill are critical - the models have NO airspeed while on the ground in the pits...
3) CL Racing (and Combat) operation is a pretty constant-RPM condition. Loads conditions dn not vary drastically as they would in throttled (CL Scale or Carrier), or maneuver loaded (CLPA) use. Load and speed changes vary heat generation more than steady load and RPM conditions. (In CL Combat, the engines seem optimized for the energetic maneuvering involved, and audibly go over the top in straight and level flight.)(Straight and level flight in a match is suicidal self-sacrifice...)
4) To be competitive, CL racing engines must be in that flat plateau of their wear lifespan where they perform well and dependably. No one wants a racer that gets a 5 lap lead on the first tankful through sheer speed, then takes 10 laps to restart and rejoin the fray... The first sign of falling off the end of the performance plateau means either accepting loss of competitive performance, or a new engine at the beginning of its useful lifespan.

Further note: PAW, for one, suggests breaking in ALL their engines on an oil-rich fuel for the first several minutes, then reducing to about 25% castor for plain bearing engines, 22%-23% for single BB engines, and 20% or so for general use 2BB engines - and less oil% for well broken-in engines used in racing.