2000 MPV noise from camshaft area

I trailered a friend's 2000 MPV home today after it began to make a loud tapping noise from somewhere in the top of the engine. The engine oil had been changed (by me) about 600 miles ago, but had been neglected for a LONG time before that change and had been driven extensively in hot weather with a very low oil quantity, sometimes too low to register on the stick. The friend's hubby is a real slacker....but that's another story. When I drained out the old oil it was gritty and thin, almost like an abrasive cutting fluid! It was dark when I got home, and I haven't tried to diagnose the noise. My first thought is that the top of the engine isn't getting any oil; sludge in an oil passage. Does anyone have any experience with cam/lifter/upper end oil problems on these vehicles? Any help appreciated!

In the light of today I now have a liittle more info...the noise is emanating from the right front of the cam cover on the right hand (rear) cylinder head of the engine. The engine starts and runs great, the oil pressure idiot light goes out immediately on engine start, the oil level is OK. The noise began about 2 minutes after I started the engine today. With an engine stethoscope, the noise seems to come from just behind the cam sprocket, about where I think the cam bearing is located. I'm still thinking that bearing may be oil starved, another possibility might be cam chain noise that is transmitted to the cylinder head at the first bearing journal.
As always, help and opinion from you gurus will be appreciated!