The Lucky Horseshoes you speak of were a marketing symbol used by Gulf Oil in the sixies. If I remember correctly, the slogan that accompanied the horseshoes was "get that kick" or "put a kick in your tank" - something to that effect. We had a Gulf dealer just around the corner from where I grew up, and they used to give away the horseshoe stickers away for free. In fact, I had one of the horseshoe stickers on the banana seat of my Schwinn Sting Ray! I also remember seeing many Mustangs sporting the stickers on their rear bumpers.
Hope this helps!