I have a 2009 Toyota Sienna XLE with the "Towing Prep" Package. I have added Air Lifts, Hitch, wiring/controller, run Mobil 1 Synthetic Oil, and added brakes to my pup). I am within the towing limits when I pull our 2004 Flagstaff 208. I am about to get the Transmission Oil Changed and thought this would be the time to add an aftermarket Transmission Cooler if I want one. For those that don't know....this generation of Sienna all have a Trans Cooler internal to the radiator. In fact...the ONLY thing that the towing prep package has that standard van does not, is an oil cooler. When I talked to my transmission shop, he said to get the dealership's recommendation. I called two dealers. The local one (that I don't trust much) simply said to go ahead and add it...that the stock cooler is only for "normal driving conditions." The second dealer I called said that the stock one is sufficient for towing a pup...part of the "towing prep" package! The master mechanic said that you can overcool the fluid and adding another cooler may even make the transmission shift.....funky. I don't have a transmission oil temp gauge....but since it's internal to the radiator...I kind of go off my engine temp gauge. I pulled the pup across country last summer....through Southern CA, NV, TX...etc. with temps up to 115 and the temp gauge never went above half way. Made me nervous and hence why I'm getting the fluid flushed and considering a secondary cooler. Any experience with Sienna's towing (particularly transmission) or thoughts?