JJ, when I purchased it the dealer said they typically replace it with another type of oil at the first change, then leave it in. His reason was he claimed the mobil 1 didn't not hold up as well as it used to in high heat. This would be typical in my case as its an air cooled unit behind the drivers seat. The dealer was one of three I visited, he sold allot of various ZTR's and very large Case farm equipment, he seemed very sincere compared to the others.
I could only imagine the difference would be the newer API codes for emissions would be more additives (zinc, coal tar, etc). I am not sure if UDT oil has to meet any of those API codes since it is used only for hydraulics and not in the crankcase, so I believe the additives would be less, therefore more oil and longer life. Besides I have allot of UDT around for my BX23 and it would just make it easier.