As stated, two issues, valve deterioration and ethanol depredation. Hardened seats and valves have eliminated the first. The second is easily finessed by using non ethanol fuel, not that hard to find, at least around here. It is more expensive, but the mileage improvement pretty much makes up for it. We use it in our modern cars as well as the Kaiser (plus every small engine we own.) Ethanol can wreack havoc with a car of any age, hygroscopic as it is. It also lowers the mileage, and the higher octane ratings it produces do not appear to be entirely legitimate. A couple back there were warnings sent out for our Audi to not use ethanol during that cold winter, as it was freezing up the fuel pumps inside the tank.
Ethanol is also an ecological disaster- net carbon footprint of production and burning exceeds that of real gasoline. Corn is for eating, not powering your car.
Stick with non ethanol and you will have no problems, assuming your valves are hardened.