1985 ford bronco 5.0

I just got a 1985 ford bronco from a friend that has been sitting in the field for about 5 years..I threw in a new battery and starter and it fires right up after it runs for a few minutes the oil pressure starts to slowly drop I am hoping that it just needs to be flushed a few times because it was all chunky when I put oil in it.... I am hoping to turn it into a pretty cool 4x4 but I have to start with engine first..so hints ideas and concerns will always be greatly appreciated.