XJet

Less oil means more methanol (in a given volume of fuel) and more methanol means more power. It's not actually that simple but basically oil doesn't add any energy to the combustion so it's reasonably true.

I would not be tempted to go below 18%-20% oil content on any modern 2-stroke (or 4-stroke) model aircraft glowplug engine (large SuperTigres excepted).

While you may well get away with a 15% oil content for some time, it will only take one lean run (due to a split fuel-line, air leak around the carby or whatever) and you could ruin your engine.

Rather than a 50/50 blend of synth/castor at 15%, I'd recommend that you go for a 80/20 blend of synth/castor run at 20%. This formula will give you 16% synth and 4% total oil in your mix. The small amount of castor will provide a huge margin of safety for your engine and the synth (being less viscous) will gain you a few hundred RPMs without sacrificing total oil content.

Modern engines will run on less oil, but not for very long -- just look at the short life of model/truck nitro engines where oil percentages are sometimes less than 10% and lifetimes are measured in a few short hours.