Chevrolet Repair: Low oil pressure, chevy z71, low oil pressure

Question
I have a 1993 Chevy Z71 4x4 5.7 liter engine with an automatic trans. When I'm pulling a long hill, especially when I'm pulling my tent trailer. The lifters run dry and start to rattle alot and the oil pressure will drop to under 20 pounds. The first time it happened I thought that I had a collapsed lifter or one with a weak spring. So in my parts changer wisdom I changed the lifters,I thought that I had the problem solved. The next year on vacation the same thing happened. This time paying particular attention to what was going on, the lifters in the front of the engine were rattling and the oil pressure was low. Going back down the other side and on the flat the oil pressure comes back and the rattle goes away. It's crazy! Can my oil pump be weak, worn out or partially cloged? I only have about 120k miles. Any ideas?
 Thanks Kurt Kolb

Answer
Hi Kurt,
I think the oil level may be too low.
Are you sure you have the correct dipstick?
Assuming you do, and even the capacity matches the owners manual recommendation, I think the oil is either not deep enough in the pan sump, or there is a leak in the sump/screen tube.
Van