I'm assuming it's a diesel. If so I would use a diesel rated oil. Mine calls for 15W40 like Jim's post above. I've never used 10W30 in the winter but it's an idea.
I've heard the Hydrostat produces a little more heat than a gear tractor. So check the fluids as Doc suggested and make sure the u-joint on the PTO shaft is well greased.
Does it have the lever type or the hand tighten type? I'm assuming no hydraulic brakes? Go to eTrailer they have rebuild kits for trailers and show parts diagrams.