I'm back with an update. Thanks for all the help so far. I tried
losening a couple of the rockers with the motor running as suggested
but it didn't work; still no oil even after 30-60 seconds of running
like that. I also hooked up a trusted mechanical oil pressure gauge
and found that I do have 50psi at cold idle, so I'm ruling out oil pump
and any serious bearing trouble.

Here's a little background that may help the experts. This all started
when I took the truck on a trip much longer than we normally use it.
Typically, on the farm, it doesn't run for really more than 30minutes
at a time, but I was helping my brother move and had to drive it on a 3
hour trip, including some highway. On the way, after about 2 hours, I
started hearing a ticking in the motor. It's hard to describe the
noise; it was louder than the tick that comes from a loose rocker, but
not as loud and deep as a connecting rod problem. It's frequency was
dependant on RPM, but the noise did not get louder if the motor was
revved up; it just ticked faster. I had no idea what the cause was,
and since I was far from home anyway I figured the motor was toast and
I tried to make it as far as I could. It made it the rest of the way
to his house and back, running the same way.

I started working on the motor and found the spark plug on the #5
cylinder was smashed flat on the end, and that with a new spark plug
the piston was not working right; it skipped and removing the spark
plug wire from that spark plug made no difference in how the motor ran.
I pulled the valve cover and found only a tiny amount of oil flowing
down the rockers when it was running, went farther and pulled the
drivers side head and found the exhaust valve on #5 was broken - 1/2 of
the outer ring of the valve was gone and the piston dome was pretty
lumpy. No pieces inside.

I had the head checked and fixed, put it all back together. Now the
motor runs very smooth, but the loud ticking is still there. With the
motor running, I loosend the rocker on the #5 exhaust valve, and the
noise got less and less; with no tension on the pushrod tube (it just
going up and down but not compressing the valve, it was almost gone -
the loud ticking noise (normal valve ticking was going on though cause
it was loose). As soon as I slightly tightened the rocker the ticking
started again. I pulled the intake off but everything looks ok, but
what do I know?

So - what does this mean? Can hydraulic lifters go bad and cause this?
Could a bad lifter keep oil from going to the top of the head?

Oh, and by the way - when I took the distributor out I think I know
where the idea about the distributor came from. The channel in the
rotor housing that passes oil from the pressure port behind it to the
lifter ports on the side is not a consistent shape; one side narrows
down and is much smaller than the other side. I think if it was in 180
degrees backward it could impede the oil flow, but it didn't look like
it would block it completely on my engine. It was in 'wrong' from what
I could see - the narrower restricted side was towards the oil pressure
port in the block.

Tomorrow I will replace the lifters and spin the oil pump with a drill
to make sure oil is getting up there. If that doesn't work I do not
know what to do next.

Thanks again.