i had my oil changed at local shop, well picked it up droped another truck off to get oil changed in it to. well drove the 6.0 home and then back to pick this truck up. then i went to pick my bow up 4 miles away then some hay. well when i got to my hay man i looked and i had not oil presser and when i give it gas oil presser goes up but just sitting there it has not oil presser at all. what could it be?