I would try a different pressure gauge first, as you don't want to be doing a tear down just for a dicky pressure gauge!!
Personally, so long as it is ok whne above about 2000 rpm I would leave it alone, there is such little load at idle this pressure is no problem at all.
The ford figures assume that everyone will just buy a new engine from them, that pressure is perfectly adequate, some engines fall off the bottom of the scale on tickover, but pick up once spinning a bit faster.
Yes wrong oil, ie too thin will lose pressure as it's still thinner when hot, so will worn pump blocked strainer etc.
I would try an oil change first with at least 50 weight oil, ie 5W/50 or 10W/50 etc.
tabetha