Fluids break down - decompose, wear out, it is not only mileage it is age (how long, this is a consideration). There is significant fluid remaining in the torque converter after a draining the transmission. I don't know the percentage, but it is significant. For the money saved by not buying OEM, might consider putting the saving into flushing all the fluid, disconnect the lines at the transmission radiator, the front wheels need to be blocked up so the wheels/tires can spin, no problem if it is on a rack, put buckets under each, intake, output, I would guess you need at least 3 gallons - you can return the unused. Run the engine, with transmission engaged, then I would suspect 97+% of the fluid is replaced. This is posted on you tube I am sure, I recall, if not is sure should be, The only reason I post this is that there is a significant difference in price between replaceing a transmission on a front wheel drive and rear wheel drive, this is a consideration. Don't forget the differential fuild. As for flushing, might consider doing the same procudure for you power steering fluid.