You know, it could be that there is just too much oil in that SIG fuel. I think, but need to verify, it may be Champion 15.
15% Nitromethane
20% Lubricating Oils
(1/2 Castor, 1/2 Klotz)
Now the OS manual, and the Magnum manual state to use 20% oil so that could be why they run OK on it. I will run a tank of it in the Gemini and see how it runs, if it runs OK, then I can use up the gallon and a 1/4 I have left of it.