mixing fluids

The prior owner of the M4 071 used Napa premium hydro fluid for all the hydraulic services during the 1800 hrs he owned it with no apparent problems. I have always used Kubota Sudt for service on my other tractors. Can Sudt be mixed safely with other “compatible” fluids?

TLDR: Can I mix UTF and NH134 to top off my rear differential? I recently purchased a Ford 4630 turbo diesel and I'm performing general maintenance. The dipstick to the rear differential indicates I need to add fluid. NH/CNHI indicates NH134 fluid is recommended for the power steering...