This will undoubtedly prompt heated (pardon the pun) replies, but on the few occasions it gets seriously below freezing here in south Georgia I use the built in 5,000 watt to 10,000 watt (approx) fluid heaters on my Cat and John Deere crawlers and Kubota tractors. They work very quickly because they heat only the fluid and not all the metal around them, and in some cases I can drive while the fluid is heated.
The heaters are the relief valves in the hydraulic system. Just extend (or retract) a cylinder on the FEL or blade fully and hold the valve open by hand so that all the pump output flows through the relief valve. Virtually all the power of the pump is turned into heat. For example, the 9gpm pump on my Kubota Grand L 4330 produces over 10,000 watts of heat. That warms up 15 (?) gal of UDT enough to function in just a few minutes.
Does it hurt anything? I don't think so. In fact, Caterpillar and John Deere recommend it as the method to warm up the fluid for pump testing. I guess it may wear the pump a little, but in the overall scheme of things it can't be much.
Furthermore, in most small tractors the power steering is powered by the single hydraulic pump, and obtains its fluid through a spool type priority flow control valve. That spool valve restricts the flow of the excess fluid (say 8 gpm out of a 9 gpm flow) in order to force the 1 gpm of fluid into the power steering unit by reducing the orifice through which the excess fluid flows and increasing pressure up to the system limit. That's the buzzing noise you hear when the front wheels are turned to lock.
The only possible harm that I know of is breaking of the long stream polymers used to improve the viscosity index of multi-viscosity oils. The test for the viscosity maintaining capability of a multi-viscosity oil is to pump the oil at high pressure and temperature through a small orifice, because that breaks up the chains of long stream polymers and turns, say, an SAE 15W-40 into a 15W-20. I don't know, but I suppose the relief valve could do the same.
But I don't think my short time of running the fluid through the relief valve adds much to the VI improver breakdown that might be occurring in the power steering system and in the FEL when heavy loads are lowered slowly (which makes the return fluid flow through the narrow valve opening at high pressure). And wet brakes (and clutches in the Glide Shift Transmission) subject the fluid to shear stresses every time they are used.
But the hydraulic oils I use are single viscosity anyway (John Deere J20 and Cat TO-4 20) so viscosity breakdown is not an issue with me.
Course last three snows here were in 1973, 1977, and 1993. So I don't have to heat the fluid a lot.
