How do you change hydraulic oil on your bad boy mower?

I'm still trying to figure it out, but here is what I have so far. I put a drain pan under each filter and then removed the filter so it could drain. once a side has drained, replace with a new filter and repeat for the other side. Then remove fill/vent hex or allen nut at the top of the casing near where the reservoir tube fits onto the casing.(Look hard. It blends in. Its on the inside, center of the casing)

My only question now is how much to add. I'm guessing about 4 to 5 quarts total came out. Also, I'm not sure if you add the new 20w-50 motor oil to the reservoir and let it drain into the casing or somehow add it through the vent holes. Good luck