I'm sure someone will have something to say about it, but I didn't replace mine until the bike had 160,000+miles showing on the odometer...can't tell you how much oil as the oil volume had to be adjusted for the suspension's ability to lower to within one inch of totally bottomed out.
At present, despite the forks being fitted with shorter springs and air fittings so I could run them as air-assisted (pressurized to between 60 and 70 PSI) for several months has not revealed any leaks.
I brought up the service manual and it says to "inspect" the forks at 7500mi or 24 month intervals. that includes leaks, damage and what not. I wouldn't bother with the fork oil unless there is something wrong or if you want to go for a heavier viscosity.
I went with a heavier viscosity, this give the forks a much harsher ride but I did it in order to reduce some brake dive.