If you put unleaded gas in an old car, you can get a fuel additive to protect the engine from the harsher fuel. You could also see how your car performs on unleaded petrol in a sensible way, for example not driving it long distances. Door to Door Car Carrying specialises in interstate car transportation for classic cars, modern cars, caravans and boats. Wiki says "Though Coleman fuel has an octane rating of 50 to 55 and a flammability similar to gasoline, it has none of the additives found in modern gasoline and cannot be used as a substitute for gasoline, kerosene or diesel fuel in modern engines.
It depends on the car. No, you can not mess up your car by putting unleaded plus in it. Tetraethyl lead was added to fuel to reduce the volatility of the gas and increase the octane rating. If you own a classic car built for leaded fuel, you can either convert it, or take the easier option of using a standard fuel and adding lead to it with an additive like Redex Lead Replacement. Any car that requires unleaded fuel will have warning labels on it. On the other hand, many classic-car owners argue that lead substitute can't hurt your engine and may help reduce any risk of using unleaded fuel in an engine intended for leaded gasoline.
On older cars however ( as noted by the person with the '66 Mustang ), they did have softer valve guides and do require a lead substitute to be added to the unleaded fuel to operate properly. Just a little bit of lead in gasoline can raise octane by about 20 octane numbers. Racing fuel is leaded by the way. However, it will almost instantly ruin your catalytic converter which will cost hundreds to replace. No engine built before that time can possibly need leaded gas or lead addatives, because they weren't designed to take advantage of it.
Unleaded simply and literally means that it does not contain lead. However, you can limit the damage to your engine by leaving it switched off and your keys well away from the ignition. If you have used it all then there is nothing to do besides drive it. This is why you don’t see many unleaded racing fuels with octane ratings much over 100, while their leaded counterparts can get close to 120 octane. Usually there's a label at the fuel filler, along with a restricted opening that keeps you from putting too big of a tube down there (the old leaded pumps had a larger nozzle). Some cars, labelled as flex-fuel vehicles or FFV can switch between E85 and normal pump gas without any issues, but if you accidentally fill your non-FFV car with E85, you may notice some issues.