We have a lot of equipment at work that uses hydraulic fluid. The vast majority of the stuff I us AW46 oil in, non brand specific. I haven't seen a major benefit from synthetic hydraulic oils in most cases. Some of our stuff that operates in very cold weather, 25*F and lower consistently, I've run syn oil in. Also if the equipment runs very hot, such as one truck that has a badly designed valve body gets synthetic because it stands up to the EXTREMELY high sump temps. I can't justify the cost of synthetic hydraulic oil most times.
I put desiccant breather caps on all our Dumps, loaders, dozers, excavators and backhoe's. It's really made a big difference as all of those pieces of equipment live in VERY wet environments.
I'd also recommend installing a return side filter on those trucks. It's easy to do and would really help the spools and packings live a happier, longer life.