Burning the Auto fuel without the STC is a No-No, but like Gary said the alcahol will eat the seal and hoses. I just spent two weeks flying a 1956 175 with the O-300 and we loaded it up with three full sized adults and 40 pounds of bags, it climbed steady at 500FPM in that hot, humid Alabama day. Also no issue on takeoff to clear those tall trees at the end of the runway. Just my view.
