I thought someone would have replied by now...
Its a long time since we had one of these, but the basics should be the same.
On the base of the
oil tank in front of the radiator should be a drain plug. slacken this but don't take it out yet.
There is a steering ram on the left side of the tractor. From memory, which may be wrong, Undo the lower flexible rubber pipe and put it in a bucket and start the tractor. if the oil pours out of the ram it was the wrong pipe and you will have a mess.
When oil stops coming out stop the engine immediatly and drain the rest of the oil out through the bottom of the tank and let it drip until most of the oil is out.
Replace the pipe on the ram and the plug on the tank.
Later models had a filter under the big lid on the tank. If yours has, remove it and wash it in petrol and then dry and replace.
Then fill to level on dipstick, which can take quite a long time because of the small mesh filler filter.
Run engine and turn steering fron lock to lock to refill pipes then top up again. this won't change all the oil since there will be some left in the ram, but if you'r ultra fussy, change it again after a few hours work. Oil should be ATF , any brand.