There is a lot to be said about this.
It's probably a worn engine; cam bearings/main bearings/oil pump/etc.
The first thing to do is verify the oil pressure with a manual gauge to rule out the stock gauge just reading low. I bet the gauge is fine though.
You can put in new oil pump gears (costs about 15 bucks) which might help.
Get some 15w40 Rotella oil - that will help.
You can go 'all out' and remove the entire timing cover and do lots of mods to it to boost oil pressure.
http://amcramblermarlin.1colony.com/photo2_1.html
My '88 AMC 360 in the waggy had about 140k miles and lost nearly all oil pressure. I did all the mods and they helped, but I eventually swapped in a good 401.