If the bike runs "good" with a slow jet you think is too large, why don't you think it would run better with a properly-sized jet? I think trying to adjust for improper jetting by playing with your oil ratio is a bad idea.
As to your question why increasing the amount of oil leans the mixture, the thickness of the oil has nothing to do with it. It is the temperature of the combustion chamber that matters. If the bike is jetted properly, the combustion chamber will be hot enough to burn the oil, so you will have little to no spooge.
The fuel's change of phase from a liquid to a gas (evaporation) lowers the temperature of the mixture. Consequently, excess fuel (richness) will overly cool the cylinder, dropping the temperature, resulting in incomplete combustion of the oil. Conversely, too little fuel (excessive leaness) results in inadequate cooling, causing excessive combustion heat. This is what usually causes seizures caused by lean jetting.
Proper jetting in a two stroke makes a huge difference in power and throttle response. Why don't you just try a one-step leaner slow jet and adjust the air screw, then see how it runs? It would cost less than $10. You can adjust your clip position for free. These are easy changes that will likey yield you a lot of bang for the buck.