Acceleration control rings, exhaust choking and engine heating, and fuel and oil mixes

Hello, I have an 0.049 RC Phoenix engine from EX Model Engines which I got for a flying wing (it's a long term project at this point).  This engine has a throttle ring, a.k.a an acceleration control ring, on it and I have read that these things can cause the engine to overheat at low speeds and at idle since.  Given the way they work, this seems very likely to happen if the engine is left to idle for any extended amount of time.  I was wondering if this undesirable engine heating could be delayed, or ideally eliminated, by increasing the quantity of caster oil in the fuels that are normally used.  I was planning on using Sig Champion 25, with 25% nitro and 20% oil, 10% caster and 10% synthetic, but now I am reconsidering that choice.  I am thinking of increasing the caster oil content to 12%-15%.  I am leaning towards 12% and possibly something with less nitro in it.  For now, let's suppose I'll use the Sig Champion 25 fuel.  I am aware that this will lead to the build-up of a varnish at a faster rate and so require more frequent engine cleaning, but if it offers a protective advantage, then that price is acceptable.  I would be interested in your opinions on this.  What oil percentage do you think would be desirable?

The increased caster oil quantity, since it will act as a coolant, is going to affect the idle.  Specifically, it should make slower idle speed less stable, forcing higher idling speeds.  It will also cool the glow head/plug at a faster rate.  What have you done to counteract this problem?  

Thanks

I use home blended 60/20/20 fuel with all-castor and have very good idle without engine overheating.