RE: Model 12 Winchester?
What is the constriction of the barrel?
I'd stick with lead shot.. no doubt. If she is as well kept as you speak of her.. then she is no doubt ready for action. A finer pump gun will never be built. The 70s model 870 Wingmasters were CLOSE.... but still not a model 12.
I'd pick up some Winchester Supremes, some Remington Premiers and a box of Federals with Flite Control wads. Put her to paper at 30 yards three times with each... see what you think. I personally have always shot #6s. Others have their preferences, go with what you see on your pattern paper. Take care of that gun... and don't let your buddies pry it away from you.