Dana,

I have used the Tractor Supply version of Universal Tractor Hydraulic Fluid and they tend to carry two brands in most stores; Traveller and Mystic. May leak slightly more around old seals that are already showing a slight leak (thinner fluid) but both of the above work well down here, even with over 70 days above 100F in our area last year. I see recently that TSC now offers a "High Mileage" type of the above, for "older tractors that required straight oil originally" and it is supposed to be a bit thicker viscosity. Whatever you use, make sure it shows "Allis Chalmers 821" on the label where compatibility of other brands are listed. I run Traveller or Mystic in all eight, including three D15II's, the D17IV and the D19D and never had an issue. (the D19D is the unit I brought down from Herkimer that we talked about last year)