1994 3.9 5 speed. I took the old beast on a road trip up north, about 175 miles. Most speeds were between 50-65 mph. I hesitate to drive it at the 70+ mph due to the high odometer of 204k.

One thing I noticed that when at these speeds, the oil pressure gauge was in the higher range, then on occasion, it would drop a little bit. then go up?

What would cause that??

 

Normal. Higher RPM means the pump is pumping faster, so, pressure increases.

 

and for gods sale drive the truck. Lol. Miles don't mean nothing. Theres nothing that's going to happen to it at 75mph that wouldn't happen at 55. (With in reason of course) babying a truck is a good way to kill it. Not saying abuse it and run it hard though either.