Go to your local navistar dealer for a dip stick....uh....about your oil pressure...try the oil pressure sender.
Question 1: After your engine is warm, does the oil pressure gauge return into the normal range?
Question 2: If you answered yes to Q1 does it return to low when you stop at a stop light or idle again?
Those senders are kind of a joke...all the gauge is an idiot light with a resistor and a needle. After your engine makes 7psi of oil pressure the needle will just settle out there somewhere and you feel all happy cuz it appears to you that you have plenty, when in truth you could be running 3250 rpm with 8psi and it would say you have plenty of oil pressure. lol. Unfortunately to me it sounds like you just get dangerously low oil pressure after your engine warms to operating temps? I hope this isnt the case. Good luck with the van dude.