I've got some Morgan 30% helicopter fuel I added some castor to and that's what I usually use. I've flown Cox .049's on regular old Morgan Omega 15% fuel as well. They said basically any plane flying a Cox engine counts, and you can throw a quick PT-19 together from the attached plans. This year's event is Friday, Saturday and Sunday, September 22, 23 & 24th.
https://www.coxengineforum.com/t7189...-it-day-thread