Id refer a transmission oil cooler for an automatic transmission. if u reduce the ATF temperature the oil will last longer in a transmission. i have one in my car and the difference was substantial.. alot smoother and crisp gear changes. alot of cars have a transmission cooler as a unit in the radiator where the coolant is the actual cooler and the atf flows thru in a tank. coolant being at around 90'C at operating temp will cool atf quite well which can run up to 160'C. a separate transmission cooler is added say a frount mount cooler it will reduce the Auto fluids temp even more.
id say do it as preventative, but if its a manual gearbox dont even think about it, anyhow if its manual u will have to source a pump to get the oil moving as well.
Engine oil cooler, id say get one of ur doing some serious track work, street driving i wouldnt even look at it unless ur engine is highly modified.