440 losing oil pressure

here is my situation .. got a car I'm thinking of buying the car has a built 440 maybe 4000 miles on it ..oil pressure has been fine around 70/80 lbs while driving 65 at idle.. the guy who owns it called me as we are negotiating the price to tell me the car is acting up ..it is at 60lbs at idle and while cruising but when he accelerates it DROPS to 20lbs. ..there is no noise coming from drive train at all.. and car runs fine ..it just loses pressure .. the key to me was he JUST changed the oil and filter ..is this what happens if a filter (fram) collapses... I have heard of filters collapsing and was wondering if this is the type of result... especially since he just change the filter