I can't say for sure for your model but mine actually had a Product Improvement Bulletin from Kubota that replaced the sight glass with a dip stick and Kubota added 1.5 gallons more. This was to get the oil level up above the ratching cams (long store...).
There is roughly 3.25 gallons of extra space (give or take) in the oil reserve tank. I run my L2800 with a gallon over and never had the slightest issue. Except that I use an extra gallon that I don't really need to.
How much oil totally is in your hydraulic system? My guess is in all the lines and cylinders, you are using a gallon the rest is just chilling in the tank... That's my take.
I've lost 3 gallons and was still able to drive it back to the house to fill it up... also, no problem. My model uses 7.5 gallons (IIRC).