I have a 01 QX4 that I just installed a hitch on and towed my 19ft ski boat (about 4,000 lbs) with it for the first time. I was impressed with how well it towed. I have been told different things from different people regarding the use of overdrive while towing. Should I always have the overdrive off when towing or is it OK to have it on while on level roads and down hills?

it is recommended to leave it off.