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In general, the hydraulic fluid will not "freeze", but its viscosity will increase significantly with decreasing temps. Most northern TBN'ers will warm up the tractor for 10 minutes before significant use, and will cycle their cylinders part way through to help circulate the oil and get it warmed up. Thick oil will cause sluggish response and whiney pumps.
You will have problems with diesel fuel in colder temperatures. While it doesn't "freeze" to a solid, the paraffins and the larger complex molecules in diesel fuel will allow it to "gel", and it will not flow through filters or fuel injectors in that state. This problem can be alleviated by using fuel purchased in the proper season (i.e., don't try to use diesel purchased in the summer for your winter tractor work) since the refiners will alter the mixture in the diesel fuel on a seasonal basis. Winter fuels will have a lower gel temperature. There are also additives that you can add to your diesel fuel to prevent gelling. Some people advocate using kerosene to "thin out" the fuel. Too much kerosene, however, decreases the lubricity (the lubricating properties) in diesel fuel, leading to engine damage.
Explore the wonderful "search" function on the top of the page, and search in the "Oil, Fuel and Lubricants" forums, search for "Diesel gel cold kerosene", with search option "and". You'll find a lot of facts and opinions regarding cold tractors and diesel fuels.
