318 low oil pressure

The original oil that was changed in favor of Mobil 10w40 was Mobil non synthetic 10w30, the first filter that was on there before the 10w40 change was a parts master filter, now has a motorcraft filter. No dark oil(clear), clumps, or even thick spots. And the milage is 13,000 miles(not a typo), I bought it from my neighbor who was the original owner who verified the mileage and I know the mechanic who worked on it before I got it and he also verified the milage. As for pulling the pan and covers, I will post pics of when I get around to doing it.
Again thank you guys for the help.