That's one of those "almost, but not quite" things Pete.
If I remember my thread history, NPT is basically the same thread size and pitch as UNF, and was designed originally to seal either against a tapered NPT or a parallel UNF thread, of course it prefers the NPT.
As an example, think about the headless bolts on the Dodge rear springs. They are fitted with NPT grease fittings, but the extractor tool is a plain UNF thread of the same diameter - you just screw it in till it jams then rotate the bolt out.
All the tapered pipe fittings on the GMC or the Dodge will be NPT I think, with one silly exception. On the civilian-style oil pressure gauges the thread into the back of the gauge itself is some oddball non-NPT thread. The absolute best source for all this info is the ORD 7-8-9 Parts List for the vehicle, where all the thread sizes of hose, tube, and fittings are laid out plain and clear.