When cold at idle i have 40psi
It used to be when hot cruising i got 40psi now it's 30-35
But hot idle has always been 15-18psi at 600RPM

Could the pressure relief spring be getting weak?
I know these are not High pressure engines but why would it change over the last little bit?

Also do 351W valve seals fit a 300?

 

Seems to me oil filter needs replaced...

Verify Oil Level, oil type/brand, use a mechanical gauge to verify readings.

 

I did an oilchange 500 miles ago with Motorcraft FL1A filter and Rotella 15W40

but it is possible that the filter is clogged. The truck sat for 10 years and i have flushed alot of crap out of it. So there could be more

I have a Mechanical Guage

 

Original Ford Oil Pump use a Plug to keep in place the regulator spring, once I found an engine having low oil pressure and I notice it was caused by the plug, in someway it was moved back about 1/4" and of course the spring lost some tension...

 

ive had problems with the sending unit showing low or no oil pressure, after replacing the sending unit in 3 different trucks & vans Im used to seeing these fail. cheap under $10 at the parts stores

 

it has a mechanical Guage

 

Did you change oil and filter???
Did you test with a know good pressure gauge???

 

not yet
i only drive 100 mile a week i'm going to change the oil this weekend

 

when i bought my six it had low oil pressure. turned out the oil pickup screen was clogged. to try and fix it without dropping oil pan maybe try heavy duty oil cleaner and change the oil a couple of times. just my 2 cents.

 

that's what i'm thinking
i and going to pull the pan hopefully during spring break between the 19 and the 26th to put a pan gasket on it so i'll check.
but i might have to wait till the following week because were putting up a fence and i might have to tow a bobcat with and auger bit on it. so i'll have to see if were going to do that first.