The rear end/differential and the hydraulics have a common sump on that tractor. The transmission is a separate sump. Universal Tractor Fluid (UTF) that meets the Ford/New Holland 134D spec should be used in both. The plug on the right side of the rear center housing should be just the level hole. The plug on the side of the transmission is for both filling and checking the level. The fill plug for the rear/hydraulics should be on top of the differential housing on the right side. If you can't find a fill plug on top, you can use the level check hole on the side, it's just slower and potentially messier to fill it that way if you don't have to.