What tractor and loader are we playing with?
Define "warm", as in degrees?
Is it a 540 RPM pump on a 540 RPM PTO? (You say you recently bought it, one never knows what a P.O. might have adapted.)
A restriction of some sort in the line from the pump to the valve (or in the line from the valve to the tank if it's capable of withstanding pressure) would cause heating.
Also, if the loader valve happens to be a closed-center valve (or a convertible valve set up for closed center operation) the bypass would be operating continuously, heating the oil.