I also did my first oil change on the SD as soon as I brought it home, (second hand as well)
and as it is stated on the manual, it is 3L in order to leave the level right in the middle...
The thing is that you need to have some faith, as soon as you pour in 3L (2,5 first, turn on, and 0,5 later) the level will be below the min mark, at this point, wash your hands off from all that Motorex (hard to get rid of) oil, and take the bike for a spin. Be generous with the throttle...gas station people need to make a living as well.
After spinning the wheel in turns and making a few wheelies, turn the bike off, let it rest 1min and you'll see how the level reaches the medium.