I bought a 1990 F250, with 70,000 original miles. The truck was used primarily to pull stock trailers for short distances. The oil hadn't been Changed since 2008, and during that time only 1800 miles had been put on the truck. I changed the oil the second I got it home. It ran fine for about a month. Now, after driving it a relatively short distance the oil pressure drops to nothing anytime the rpms drop below 1000. Can't hear any knocking, however it does sound a little hollow when the pressure drops out. (If that makes any sense) I had the sending unit changed and it made no difference. My next step will be to change the oil pump. I hate to keep pouring money into it if there's know hope. I don't claim to be a mechanic any help would be appreciated. Thanks!

 

This does sound unfortunate. Start by doing an old fashoined oil pressure test by taking out the sender and connecting a test gauge.
This might get ugly quick, so let us know.