I use Mobil Delvac 1, it's a 5w40 synthetic made for diesels. (not regular Delvac, although a good oil, Delvac 1 is far superior)
In the cold, they start great! Great soot control, good hot weather performance and viscosity retainment...outstanding longevity, it's a top notch oil. $29/gallon at most truck stops. I change my tractors (L35 and G5200) once a year.
I also use in my newer Cummins and my older chevy 6.5TD which has 280kmi on the clock .
For hydraulic oil, can't beat super UDT2. While expensive, it shouldn't need changed all that much, kubota's done their homework that stuff, cold weather performance is outstanding, I've never had any issue with this stuff. I was having issues with a hydrostat trans in a G4200 Iused to own, changed oil to this....and after a few 10's of feet, issues went away, change oil and filter again to be sure all that old crap was out and havn't had an issue since, well, I sold that one a year or so ago, but it was working perfectly.