This will get a lot of opinions flowing, flowing much better than the oil on a cold day!
Originally they came with heavy oil, like 90 weight. Though that was good for the gears, it didn't work well with the hydraulics in cold weather.
Some have gone to 10w30, others with light hydraulic oil.
I was looking through the different options Tractor Supply had to offer, came across one that said "For older tractors with hydraulics combined with transmission oil." or something like that. Not sure what is in it exactly, didn't say, so I went ahead with it. Price was right, so far it seems to be working well.