97 sierra with over 150K miles, im almost positive its never been changed.. im getting alot of different suggestions, some people say because its so old if i flush it ill end up messing up more in the long run because of all the residue, etc, but some people say just dropiing the pan and changing the filter only does 60% of the oil and isnt the best for a tranny if its old and stuff, so does anyone have any suggestions thatll set me strait? im so confused on it.