Oil Pressure

My oil pressure gauge is reading in between o and 20. obviously this is not right but is getting worse and worse. My mechanic said a while back that thicker oil is necessary with older vehicles. I tried that and it worked for some time. But now just lately when i start it up it makes a knocking sound. Any advice? thanks.

The first thing i think about when i read your tread is thet you have some bearrings wearn out. Or if the oil pump has a pressure valve that has get stucked in open position.

it doesnt knock when i start going. only when i start it up and a little bit when stopped.

Need to verify the gauge reading by temporarily adding a mechanical gauge.  Is that the pressure at idle?  Does it go up to about 40 psi at normal speeds?
 
Is it a hard knock or more like a ticking like a sticky lifter makes?

i brought it to the mechanic and he said it sounds like bad crankshaft bearings. So it looks like ill be selling it. Want it? Make an offer.

I had a car that was diagnosed with the same thing because of low oil pressure.  Turns out it was only a stopped up oil pickup in the crankcase.