I have a PC138USLC-10 that was fine until the cold weather hit. The time temps hit 0F the operator called and said functions were really weak. Took a look at pressures is it was only making about 1000psi. Decided to get it back to the shop, when the truck driver went out to pick it up, the case for the hydraulic pump cracked. I don't doubt that the pump had been failing but I am wondering if the machine should of been warmed up before trying to track with it to keep the pump from cracking. Also had a Guzzler vac truck that destroyed the PTO for the hydraulic pump and I'm wondering if they tried to operate the hydraulic functions before the oil temp came up. Komatsu and Guzzler don't recommend any different hydraulic oil for below 0F. Their spec show what is used, should be used to -4F.
Can't speak for either of the mackes you mention but a lot of manufacturers specify a multigrade powertrain oil (e.g. 0W20) for hydraulic systems operating in cold ambient temperatures. The reason for this is that, despite the systems not requiring a powertrain-spec oil, AFAIK hydraulic oils are not produced in multigrade. A lot of users in Canada are on 5W20 in hydraulic systems. The table below shows min/max ambient temperature ranges in DegF.
Did the operator start the engine and let it and the whole machine warm up for 15-30 minutes first ? I don't start working a machine until I see some indication on the engine temp gage at least 1/4 off the peg. Exercise the systems slowly as well. or was it "Kick tire, light fire, push go button" ?