All U.K. Mogas is unleaded now and has been for some time. Comparing aero piston engines with automotive ones is not easy. The requirements are very different. As most light A/C have fixed pitch propellors, high torque at low rpm is needed - try pulling away in your car in a high gear if you want to know why! The other thing is the ability to deliver full power (or a high percentage of it) indefinitely. Not many cars get driven flat out for hours on end, day after day. Engine failure in a car doesn't usually have the same implications as it would in an aeroplane... The design of the plane can also have a considerable effect on the power required. 100hp pulls 2 people along at around 85 knots in a C150, or 4 people at over 100 knots in a modern design like the Dyn Aero MCR 4S.