Greg
every mechanical moving coupling needs some lubrication . Steresses and motions differ for each part and for each moment. So if you use a SINGLE OIL for lubrification you have to compromise.Worse, temperatures are different coupling for coupling.
If we deal with not extremely fit MOVING parts, 5-15 centistokes are a limit value that allows for making joints not mechanically and dimensionally constrained by viscosity, but only from structural needs.
It is obvious that every mechanical maker prefers not to waste material for allowing the use of low viscosity lubricants, unless constrained to. Model engines makers do not escape from this rule. Ordinary materials are used, in order to make engines reasonably cheap ( do not mention D-STAR, pls!) .
Pls note here I am speaking of plain lubrication, not of boundary lubrication since this interests zones and moments out of my rough knowledge.
In internal parts, where temperature does not exceed 100°C, the viscosity values needed are ordinarily obtained by straight oils sae 20-30. This level of WORKING viscosity is easily obtained in any two strokes gasoline engine where fuel EVAPORATES ENTIRELY after a few seconds run. This NEVER happens in model engines, so the poor lubricant has to work in diluted form.
Model engine mechanical couplings do NOT escape friction laws, and ordinary remedy to them, that is oil lubrication. BUT UNFORTUNATELY THE OIL RUNNING IN A MODEL ENGINE crankase is diluted OUT OF ANY USUAL TERM, lowering considerably the USEFUL viscosity .
In order to keep viscosity reasonable one has to rise the percentage of oil in fuel and its viscosity.
Now remember, DIESELS WORK WITH KEROSENE, that DOES NOT EVAPORATE AT ALL IN THE ENTIRE CRANKASE, (notwithstanding your hopes!) since a mere fraction of it, BY SURE less than 10%, is allowed to evaporate WITHIN the crankase in one turn and UNDER 200°C. This leads to an higher need of lubricant in the mix.
Ugo