Newer paint technology versus older paint technology. AU replaced AE. Acrylic urethane is the better paint but will cost a little more money. But is should last longer. I used to paint Ford Vans for an electrical contractor using Martin Senour Acrylic enamel but with what they called Catyl-Add which is basically a urethane catalyst. They looked good for work vans. I'd have to grab some of my old paint chip books, but in the late 60's they should have been using acrylic enamel, phasing out acrylic lacquer. If your color is a solid color, non-metallic, then just get a good single stage urethane and do the whole car in it, if it is metallic, then you can do the engine compartment and jambs with a single stage and then base/clear the car, or just stick with BC/CC for everything. What brands of paint does your local paint jobber carry???