I’m looking into them a bit, and after reading 1000 posts on forums and talking with both guys, I think it’s fine to have the fan kick on with the pump, but I would strive for a lower temperature in the box. 180 seems hot to me for oil.
I have a lot of hydrostatic machines, and the ones that perform the best for the longest keep the oil 110 to 130. In addition the Mack trucks have a transmission and rear axle temp gauges, and the transmission will typically run 140 at the high end, and the rear driving axle will run 110-130, rear coasting will run 110 or so, summertime temps.
In addition to all that, I would double check running the tank into the top of the black box, I think it should be the fill port, with a way to keep the black box from being completely filled. IIRC there are seal problems along with overheating from being a full case.
All just my opinion though, no direct experience with the underdrives. I’ll look for those posts.