Install a hdy coil heater from the radiator on your trucks engine, even if you parked it inside the long drive to the job on cold air will make your oil stiff, if you do install a heater be sure to put by pass valves in so in summer you can cut it off so you pil want get to hot.Pumpjockey can give better advice on this problem.

are you sure you haven´t any condensation water in the hydraulic system, wich is frozen to ice when it´s freezingtemp? Try to park the pump inside a warm garage over a weekend, and then the water is  under the oil in the hydaulic-tank. Try then to open the bottomplug to the tank and let out a little what´s in the bottom of the tank. I´ve had the same problem, and the problem was condensation water.

I have a shut off valve on my aggitator feed hose. It shuts down the oil flow and heats up the oil. Get to the job early and let it warm.

No, the oil won't boil at that temp.  All you need is a small coil or loop to add some heat to the tank.  Arctic Fox has them, if you don't want to build one yourself, or have one built:  http://www.arctic-fox.com/sitepages/pid21.php 

You likely have some condensation in the system, is your oil milky?  Do you drain some oil off the bottom of the tank at least weekly if you are busy?  Everyday in the winter for sure.

There are filters that are hydrophilic, they will remove water from the oil, but not great amounts.  http://www.zinga.com/index.aspx?page=series&categoryid=7

A Caterpillar or other large equipment shop that has a filter cart can cycle the oil through multiple filters to remove dirt as well as water.  It's best to do this while you can operate the machine, that way the oil contained in the cylinders and the rest of the system will be cleaned as well.

Perhaps a complete oil change is in order - not a cheap option, but it's the lifeblood of your machine.  Hydraulic pumps and other components are MUCH more expensive than that oil.

Windchill is not a factor or so I've been told. BRRRRR

It's only a factor in the case that if the unit was kept in a warm shop and the rate at which it cools the tank and oil off during travel until it reaches the ambient temperature.

A factor to consider is the stack valves - I installed a set of low-wattage (125 watt) adhesive 120 volt pad heaters to my valves to get them to operate more predictably, the kicker was, I had to have a power source nearby. I once ran 300 feet of extension cord - at 125 watts, it only took 2 1/2 amps of so, so voltage loss really wasn't much of a factor, 12 volt heaters are available too.  http://www.padheaters.com/

this is what i do.I live in northern Ontario Canada (very very cold in winter)I run a schwing 28 gate valve(yes i know its old but purrs like a kitten) i keep my pump outside a few weeks ago got a call -30 without the wind! first thing i do is start the pump let the engine warm up then when its warm i turn the pump-on while i fill the water tank as its filling i turn the outrigger valve on in the up or down direction dont matter the thing is while its on the oil is going thrue a bipass creating heat thats warming up the tank for the boom! take about half hour to fill tank then leave to go too the job the thing is its winter you got too get there (EARLY) at least an hour before concrete is to showup! on the job i take my time to open up the boom lower gear so the seals dont burst on the cylinder. When its open i put my last elbow with out the jib hose on to heat my pipes because this cold i have seen my priming grout freeze once before it came out the end but its got to be real cold to have to do that.Once the pump is set up i turn the valve for the outrigger on again while im waiting for concrete by the time its gets on site im getting heat out of my boom tank! My main tank for the pump have not ever seen it freeze on me but the thing is when i start pumping i start slow until the oil start to warmup.Been YEARS since i froze up on-site but the MAIN thing is GIVE yourself LOTS of time before the job.My coldest job was -43 celcious plus the wind.Dont over rev your pump when you first start take your time!!!!! the most north i have pumped was at the arctic circle so winter is not an issue for me anymore.PUMP-ON

Putz has installed an Artic Fox on Canada-bound, Mack-mounted Telebelt and boom pump hydraulic tanks for years without any customer complaints. The total volume of oil in the tank is large compared to the little Artic Fox ... and thinking of this in reverse, the cold oil acts like a cooler for the fluid in the Mack system keeping the temps lower as well.

For the water removal discussion, Putz also has a patented system (on-board dedicated or filter cart) that removes the water without the expense-maintenance of a filter media :

http://www.pmawatertech.com/products/dehydrator/index.cfm

As pumpjockey says, for best water removal results using one of these systems the oil should be heated.

After I had my ass handed to me I no longer pump below zero!

Seed has about the best advice.  Below zero F (-18C) takes ALL the fun out of it.  Risks are quite a bit higher, so the resulting reward should most definitely be higher.  Price should scale up in the colder weather. (Yeah, right!!)  Unless it's a very critical job time-scale wise (for boom work) most jobs won't go, however line pumping into a heated building would likely go ahead until - 25C (-13F) or so.  But it's still a nasty job.  The only upside to failure in such cold is that frozen system is generally salvageable: http://www.youtube.com/watch?v=EU_u5lXcnUI

stonezer, I think Schwing offers hyd. tank heaters, check with them; a good way for water to come in is the chevron seal on your diff. cylinder; you need to get rid of the water in the oil and then change those chevrons

I new that Pumpjockey could help out a lot he's had lots of exp with cold weather pumping, I beleive it take about 220 degrees to boil hdy oil.

How did that 750' of line blow at the end? Or did you throttle back on the air?

did 100' took too long build up in line kept hanging rubber plug..180lbs of air ..cannon style..next 650' line.. well..1 or 2 pieces at a time..started the whole deal about 3:30 pm got dark around 6pm took till almost midnight only light we had was the light of cell phones..used 5 plugs..kept blowing them into the woods..never found them..very dangerous..the pipe was going down hill..approx 45 degree angle..heres a video my son shot of the job minus the headache of cleaning up..7 pours in total..worked our way up the hill less pipe every time..4600 lbs piston pressure..all that Elba could handle..

http://www.kinzuacountry.com/Kinzua_Viaduct.wmv

here's some more pics of the job..enjoy

http://kinzuacountry.com/wp/kinzua-train-bridge-project/