I was using normal unleaded from a car garage. The Subaru engine (EA81) is an old engine so I assume it was designed to run on the old 4 star. However, the engine has been somewhat modified to increase compression, amongst other things I don't understand. EGT temps seem pretty stable at 1400°F, which is similar to what I got with the unleaded. Max temp from the engine manual is 1650 °F, so it is well within that.