My '96 T100 3.4L shows low oil pressure when fully warm and idling at a stoplight. If i give it any gas at all, even just a few hundred RPM's, the pressure goes up to almost to halfway. I only get low oil pressure when the truck has been run a lot and it is more prevalent in the hot summer days than the cold winter days. The engine is all original with only 129,000 miles on it.

The oil pressure never completely drops off the gauge, but it hovers on the lowest reading sometimes until I get the engine moving again.

These trucks take 5W-30 oil correct? Can I try using some 10W-40 to see if that increases the oil pressure slightly? Any other suggestions?