You have 178k on the transmission and you are now considering a cooler. For some reason some people here think the cooler is the majic answer to all the Honda transmission problem. The only I would put on an after market cooler is if I was towing or someone in the family has a lead foot. What did the transmission fluid look like? If it was worse than normal I would do a 3 or 4 drain and fill to clean it up. The magnifine will help clean some but it foes not get all the fluid flowing through it as all the flow is not through the cooling tube. Was the gunk on the plug metal or just gunk?