me personally I dont think it is a big deal either way but others who know more than me have posted their preferences & why, iirc something about heating up cold fluid in the winter. I bypass the rad cooler entirely just because if there's a leak in the lower tank I'll see trans fluid in the coolant or coolant could get into the trans. I'd use the largest cooler that will fit & reportedly a "stacked plate" type cooler is more efficient