It's not substandard. It's considered temporary. I mean you gotta have something in there for it to move from assembly to stock yard. Its simply one of those unspoken manufacturer's particulars you never hear of. I heard it because I work at a tractor place or else I wouldn't have either.
It simply does not have the extent of protection ingredients of the better quality oils and is why they recommend you getting it out sooner than later.
Tractor makers use thousands of gallons of oil so as to keep costs down, they'll use a cheaper oil just to get the tractor to you. Why put in $200 oil if $50 oil will suffice for that job and wanting you to flush it out anyway to get contaminants out?
As you infer, it is impossible not to get some water in a hydraulic system. You really cannot compare your example of water infiltration however, to a tractor sitting out in the elements for who knows how long before it gets to you.
Hydraulic systems often involve small orifices, which can cause cavitation and thus, localized boiling. This cavitation has the same effects as boiling and can cause pitting damage to the components near the cavitated region so the less stuff to burn off, the less chance of damage.
90% of hydraulic system failure, relates to water in the system.
I've given you the substantiated reasons for all of this but as I've said, it's your tractor to do with whatever you wish.
