To each his own with engines. I really like OS engines. But, I own two GMS .47s and I have no compliants; a ST .61 ringed; OS .10LA; OS .25LA; the list goes on. I am at 800ft above sea level though.

In practical theory, if you use less nitro content fuel at higher altiudes your results may be better. Due to the leaner air less nitro may be needed. Just imagine having a container filled with a combustable liquid, if there isn't any oxygen in the atmosphere it will not ignite, unless of course, there are enough oxygen atoms in the said combustable liquids composition.

A guy I met here in Indiana flies helis goes around the U.S. for shows, Lou (his name) told me that he has no problems with engine performance when he adjust for the nitro content at different altitudes. I have never been able test this out though.