OK, I’m clearer now what happening with the cat. Its cruder than I’d realised. Thanks for the explanations.
Re 2-stroke oil as UCL, OK, maybe not then. In my defence:-
(a) The Chinese BMW clones I originally thought of it for are probably as far as it is possible to get from “modern” in a (current?) or at least recent production engine. (Damn, now I’ve said that I want one, I knew that would happen). IIRC they suffer from poor (splash) oiling, poor cooling (the flathead is just a slab of metal without oilways) and poor metallurgy, so I thought 2-stroke oil (and maybe water inspiration) might promote survival.
(b) My impression is that 2-stroke oil is mysteriously remarkable stuff, given that, at the dilutions its used at, its viscosity must be very close to that of petrol.
The only way I can see that it can work is if it binds to surfaces/hot spots, so that its effective concentration is much higher than its bulk dilution would suggest. I’ve read somewhere that’s how castor oil, (used of course as a race 2-stroke lubricant and in other total-loss systems, such as in a Sopwith Camel rotary engine) works, turning its thermal instability to advantage.
If that is how it works, I thought there might be a dilution level where the leaning was tolerable/insignificant, but there was still a protective effect. I’d expect this to be a higher dilution to that in a “normal” 2-stroke premix.
Of course it would be possible to richen the mixture to compensate, but maybe non-trivial in a non-carbed vehicle.