why does the oil pressure drop even with 0w40 oil in it? on 1993 GMC Yukon

it starts with about 30-40# pressure and drops down to about 20 when the engine is warm, even at freeway speeds. time for a new pump? the sender is new!

Asked by for the 1993 GMC Yukon
What is factory rec. oil for your climate zone? How many miles on engine? Did you mean '10W40' or '0W40'
the factory spec is 5W30 but Mobile Oil suggested 0W40 was a reasonable alternative and would help the situation.....seems that gm vehicles have an issue with lower oil pressure when they are hot but the change in oil has not changed the symtoms. the vehicle is a 1993, with less than 100k on it.....
You did what 'mobil; said. What does your local GM shop suggest? How about 5w30 with a quart of marvel mystery oil in place of a quart of oil.
didn't think of that, i had a crown vic a few years ago that started having the same symtoms(sp)...the car is in the recycle yard now...a fried engine because when you started the car, the oil pressure was fine, but dropped off almost immediately.....even though i am NOT a gm fan, i would prefer not to go replace a vanishing breed of good heavy hauler.....thanks