Hydriv, when ever I bring a farm tractor home that's the first thing I do is drain everything, flush and refil. I did the engine right away which was filthy black, but have been waiting on the rear end as it's clean but leaks from the directional valve. Was told there is a quad ring in there so that should be an easy fix.
Someone does have 10W hydraulic oil in it I noticed. Am torn between 15W40 and 20W50, thinking of the 20W50 as some guys said it will compensate for wear in the 43 year old hydraulic system.
Go with the 20W50. As you say, the tractor will be in the garage during the cold months. Start it up with the trans-axle in neutral. Push the travel lever forward and let it run at half throttle for ten minutes to warm the oil before putting it to work when the temps are sub-freezing.
All in all it does run good, use little oil, and has a ton of power in low gear. I know Bill from Barneveld Imp and he has been fixing me up with the parts i need. I only have a little over half an acre of lawn and a 40'x60' driveway/parking space and blow about 100' from the house to my shop so it will see some winter use. Here it gets down to sub zero on occasion, stays in the 20's for most of the winter, but I keep my tractor in my attached garage which gets some heat from the house so the tractor would be stored above freezing.
I read the articles last night from Snotrocket on oil.