I have a Fisher plow I bought new about 4 yrs ago.
My question is the hyd oil, is this just regular hyd oil or do I really need the FISHER brand of oil?

Some of the plow oils have just a little alcohol in them to help with the potential problem of icing should there be any moisture in the lines. I run a Boss and I use plow oil. Unless you're losing a lot your expense difference is nil in the long run.

Last Fisher I worked on used ATF, but it was like 10 years old so that may have changed.

I'm not sure how "different" it is, but I use Hiniker Cold Flow in mine. I think I may have added a quart or two in 20 years, so cost is a non-issue.

Manuel calls for annual drain & refill............

I use the Blue plow hydraulic oil. I've used red ATF, but found it didn't work quite as well when it was extremely cold. I drain and fill mine every year. Regular hydraulic oil is probably too thick when the temp drops below zero, and causes the hydraulics to be sluggish.

Annual drain and refill might be a good idea depending on how the plow is used.

How much road slush overflows on to the pump area? How often? How much plowing annually? Two months continuous use? Three?

The concern is water/salt/dirt into the hydraulic system. An annual flush and refill is suggested for this reason.

I am sure there are systems out there that have NEVER been flushed. Considering the cost of a pump, control valves and hoses - an annual flush and refill might be cheap insurance.

We always did an annual fluid change on our fleet trucks. We changed all of our fluids over to the Boss plow fluid. I don't remember what the difference in spec was, but in appearance it was thinner and flowed easier than atf. We used to use atf in the older meyer and western plows that we installed, but I believe they went to a proprietary fluid as well. The last literature I read stated that there were no detergents in the plow fluid, and it included some sort of anti-foaming agent.

YMMV it has been 4 years since I have worked in the shop on plows.

You don't need the Fisher brand oil, but you do want do want something intended for this application as the low temp requirements matter. I'd have no problem substituting an aircraft hydraulic oil, but why bother.