First off welcome to the world of Open Carry. What your asking is kind of interesting since they should have went over it in class but Ill try to detail it as much as possible. Signs in Oklahoma are still up in the air as to how far they can be enforced but even in the worst case scenario they simply will buy you a ticket to never being permitted on the property again and if you refuse to leave then a trespass charge. The only exceptions would be listed on Page 20 of your Open Carry manual.
Parking lots are a green area on EVERY property with exception to schools and even then you have one exception to carry there and thats to drop off and pickup kids all the while staying with your vehicle. (I strongly suggest if you do this though you CCW just because I for one dont like the inside of jails even if in the right and can get off the charge)
If your pumping gas the manager can at their choosing ask you to leave it in the car since your standing outside of it but I seriously doubt this will ever come to pass. Since this is so grey as to are you leaving it in the vehicle or not can cause lengthy court disputes I imagine most will simply tell you not to bring it in if they chose this route.
As for signage i would just leave it with "No weapons except by valid permit holders" would work best. If you get the sentence too long believe me people wont even read it. Its kind of like if you go to an amusement park do you really read all 30 rules on the sign before you go in?
Oh and I have carried for 17 years and still turn my firearm side away from people. Its not to hide my firearm though its just a tactical maneuver and its called an interogation stance.