Four speed gearbox - what's the oil capacity?

I've read a lot of tech specs that say the capacity is 1.5l of oil. I drained the box today and about 1.5l came out. No idea when it was last changed.

I refilled with fresh oil, just over 1.5l. then I looked on the forum and a lot of people say you should put in 2l.

Is 2 litres too much for an old four speed box, or will it do no real harm?

I'd rather not overfill it, but also don't want to risk running dry and damaging the gears!

fill until its running out of the filler/level hole