You will have a hard time finding 20W40. Use 20W50 instead. Much easier to locate. There is a 1/4" pipe plug with an allen head that screws into the bottom of the travel/lift valve under the checker plate floor. Clean out the hex hole thoroughly and use a socket wrench allen wrench that is pushed into the hole all the way to remove the pipe plug. You get one chance to remove this plug. If you round out the hex, you have to weld a wrench to the plug. That is the drain hole. Remove the plug, start the engine at idle and pump out the old oil until you see air spurting. Shut the engine off immediately, replace the plug and refill the reservoir. Purchase 7 quarts of oil to be on the safe side. For sure, you will use five to six of them.
Then test the tractor once again and let me know what happened. Often it is just the wrong oil in these tractors but... it may be a badly worn pump.