Oil pressure is low by the oil guage

Check the oil level in the engine, make sure your not low on engine oil. The other thing thing that may have gone bad is the oil pressure sending unit, it's located on the engine block on the passenger side by the distributor. If it's not leaking oil you could always try adding a bottle of Marvel Mystery Oil in to your engine with the engine oil. This will clean up all the oil passage way and journals and may free up the oil sending unit that may be dirty from sludge. Keep an eye on the oil level when the Mystery oil is added since it's thinner and will evaporate, so it's a good thing to add it about 600 miles before you go and have the engine oil and filter changed. Good luck and hope this helps, Auto Zone stocks the oil sending unit and can give you the step by step instructions for you to get the job done with common tools and should take you about 30 min to replace the unit it self.

If it is normal when the truck is cold,and then as the engine gets up to normal operating temp,then the oil pressure is dropping,find out why,oil pump,or engine bearing wear.