I am in the first off-season with my JD 717A. I am planning to change the hydro oil in the transmission, but am confused. The Operator's Manual says to use "JD Plus 50 15W-40 synthetic blend. I wondered if Mobil 1 15W-40 synthetic is really what is needed (I use 1 qt of Mobil 1 for my Exmark Viking w/b). I went to the dealer this morning to get a hydro filter, plus several other maintenance parts. I saw they had gallon bottles of JD Plus 50 performance oil on the pallet. When I asked at the desk, the parts person was clueless. He had a service tech come to the desk. His anwer was "we went to the manufacturer and found that 0W-30 motor oil was good, so that is what we use." Hmmmm... This seems strange to me. Why would somebody use a different weight, and different blend of oil from what the manual stated? When I pull out the dipstick of the transmission reservior, the oil looks and feels much like the 15W-40 Mobil 1 synthetic I use in my Exmark. But, I am unsure if the dealer doesn't have such an item, then should I go to the auto parts store and buy Mobil 1 15W-40 synthetic and use that product. The only API service classification comment is "SG or higher." I want to be sure. First, I don't want to damage the pumps and motors. And, the manual states that 9 qt is needed (not 1 qt like my Exmark). At more than $5.00 per quart, I don't want to spend $50 on oil that is not the right kind. The reservior on my Exmark is very small. This means the fluid runs quite hot, and I can understand the need of a synthetic. However, the reservior on the JD is much larger, with much more surface area on the tank to dissipate heat. Does anybody have some light to shed on my question? What have you used to change fluid in your Z-trak?