Hi all. My father in law has a 99 ford f150 and calls for mercon v. The truck has 120000miles and of course the trans was never serviced. He is hesitant to do it because he thinks the trans will go if we flush it. Fluid is not burnt but is discolored. I guess it would be brownish in color. Truck shifts fine. Should I flush or just drop pan, replace screen and refill? Should I just leave it alone all together? I believe it's a 4r70w. (not sure if I got the model right) if I change fluid what brand? Thanks in advance!!!