I used some on a fairly hot car in the 60s and got a perceptible power bump. I never checked it out as a way to improve mileage, but it seems like it might work oK if it isn't harmful to anything else.
Well thanks for the answers. No considerable power loss and I'll give it a shot. My truck gets 13 mpg and my Mustang gets 9-10 so any increase at all is worth it to me.