Maybe I'm a little late chiming in now, but we always used to use the Type 55, until we closed our dealership. It wasn't too long after that that I switched to using universal in all of the hydraulic systems, purely for compatability. Dad had said that the main reason for the type 55 was that it was thinner than most hydraulic oils, and since those 55 series with closed center hydraulics were gravity fed through the filter, the oil needed to flow well in cold weather. Newer tractors like the 2-135 and 2-155 use a charge pump before the filter, so the problem was eliminated there. I was more worried about thinning out the oil in my tractors with common hydraulic/transmission fluid, than I was was about the others. If it is cold out, I give the hydraulics a little time to warm up before I rev the tractor up too much, and also keep the filter changed on a regular basis. I've been this way for several years so far with no ill effects.