Explain these oil pressure reads??

I bought a 87 4.0 with 110K on the clock,drove it home about 120 miles with about 40 psi pressure on probably a 2 year old oil change of who knows what oil, changed the oil with a Penzoil filter(dont say it, I know) and 10/30 oil. fired it up and the first 50 miles or so the pressure at speeds was 70+psi then dropped to a consistant 45 on the highway for hunderds of miles . Being Im sure Penzoil filters are rebadged frams I have swapped twice now for tests sakes with 2 new Wix filters and the same happens with the same oil still in it. Runs the first 50 miles or so at 70+ psi the drops to 45 or so on the road and never goes much past that even on cold start up. Any theories on this?

Try a mechanical gauge,probably the oil sender, Mine started reading very funny and I proved it was the fault with a mechanical gauge.