94 tbird 4.6 started having a faint knocking doesn't knock when started can drive it for about 20 min then it's starts knocking oil pressure seems to be ok knock sensor or camshaft phaser

 

There could be a few reasons for what some may describe as a knocking sound.
A rod knock
Main bearing knock
Hard valve tap
Possible cracked flex plate on flywheel.
You could have good oil pressure with any of the above.
Sometimes a trained ear can be fooled too, the object is to rule them out one at a time.
There are various methods to verify some and others have to be pinned down to be sure.
If anti freeze gets on a fan belt it can produce what sounds like a knock sometimes too

With a T Bird, the question of regular oil changes shouldn't even come up,, we hope.

 

Use a stethoscope to pinpoint the area of the engine sourcing the knock.You can't rely on the dash oil pressure gauge, it's an idiot gauge which will deflect in one position whether the pressure is 6 or 40 psi. You need to use an actual pressure gauge if you want to determine actual pressure. You did not mention the weight of oil you are using.