-20 C is ~ -4F. Key questions that needs to be answered before solutions can be offered are 1.) time spend inside the freezer and 2.) the actual temperature of the freezer or the rated capacity of the freezer. I have more than once been told the lifts are operating in -30F temps when the actual temp inside was -10F or even 0F. The freezers name plate had a capacity of lowering temps to -30F which to means to me a blast or flash freeze environment- like for ice cream makers. Guess it was still too cold to go inside & read the thermomter (just a bit of humor). Any operating temperature held below 32 F can be classified a freezer. But it does make a big differnce in how to best protect the equipment & allow full function for a "reasonable" operating time.

i've had some experience with these push-pull attachments
in most cases where the sissor assy is being damaged it is usually due to the operator ramming into loads or trying to push loads. I did alot of work for Hunt-Wesson when they were in my area and almost always it was due to the operators having to pack the product into a boxcar by pushing it into place. They had to pack the product in very close quarters and it always didnt fit exactly so they would try and force it into place. This would put undue stress on the extend/retract arm and would almost always bend or break something in that mechanism.

Also there are some adjustments and pressure settings that need to be checked to make sure the unit is not operating overpressure. Check the rod adjustment on the extend/retract cylinder and also make sure the forks do not have any "curls" on the end of the forks from impacts, these can cause the loads not to slide properly and can cause the load to hang up.
the leaking is due to the cold environment shrinking the seals in the cylinder heads. You should contact cascade and see if they offer any type of special seal kits for freezer applications. I dont think they do though, you may have to find a "seal making" vendor that can make seals and have some custom seals made specifically for cold environments.

Changing the oil in the lift to a freezer type oil might help somewhat but i dont think it will totally. Chances are the lift already has freezer oil in it, this should have been factored into the options when the lifts were purchased.

I would suggest you contact a Cascade representative & your local factory authorized dealer(s) for the equipment you are operating on these issues. You will need to verify some things like 1.) What make(s), models, year & serial numbers of lifts you are using with complete specs (were the units order with any "special" freezer package or just minimal cold storage features. 2.) What lubricant is in the lift rucks hydraulic system. 3.) Service intervals of equipment & who does it - PM & repair frequency, especially the hydraulic system. 4.) Average amount of time per each hour the units are operated in the freezer & out of the freezer & length of work shifts.
Often a ATF type fluid is used in lieu of hyraulic fluid in freezer applications where the lifts remain in the freezer for extended period & teh whole machine get really cold soaked but when the come out into ambient temps of say 60-70 F the fluid is so thin low viscosity it leaks past the seals. Cold soaked steel will break if you have "Cowboys" but a "stronger" pin might cause a more expensive part to break - again Cascade might be the best source for resolution and if you have "Cowboys" try to verify if you do vs a "maybe". If you definitely do, management needs to take corrective actions to fix - no piece of equipment can live up to abuse at the competitive prices customers are willing to pay.