take it to a trusted small garage, pay the guy $25 to change the oil, and ask him how many it took when he's done.
might be worth the investment.
also, look on the ends of doors and inside the door frames, on top of the radiator, oil filler cap, fenders under the hood (or in there somewhere). Most cars I've owned have it written on something obvious on the car itself. Of course I've never owned a custom coach... just factory bodied automobiles.