That may just be that gauge to be honest, or it may be 100% normal. I can remember it reading as high as 85 at idle when cold using Mobil 1. It would spike when cold and accelerating too. Sea Foam is a solvent (not a lubricant) and I'd expect that running it for too long would cause accelerated wear and thus lower than normal pressure. Honestly before you use that gauge for more than a "It's good" or "It'd bad" gauge I'd check the calibration. I'm not sure that it is wrong, but I don't know that is is correct either.

P.S. I talked to my dad, and his 98 Trooper with the 3.5 and factory oil pressure gauge reads somewhere around 85psi at cold idle.

-Tad