my truck ('88 f-150 w/ 351w) has been doing EXACTLY the same thing since ~115,000 miles. your oil pressure is not really 0 psi though...i unscrewed the oil pressure sender from the block and screwed in a snap on mechanical oil pressure gague...it read ~ 5psi at hot idle with the tranny in drive and the parking brake on. Also, there apears to be an electrical problem of some sort w/ my gague. With the headlights on and the blower motor on full, the gague reads significantly higher than with them off....

I think the oil pressure problem is a combination of worn bearings, a worn oil pump and a clogged pickup screen....pulling the oil pan will be a pain in the butt w/o pulling the motor first, which really isn't so bad if you've got a cherry picker and a friend helping.

-John