I am also interested as i just got an 03 txt and felt the same when i filled the manual amount, but if i run the bike for a bit and then check the sight glass it sits at the correct level - not sure if this is how it should be read though
the jim snell page is gone... but this adv rider page says what i kinda remembered.. the pro used less oil... i was remembering 380 cc, but it shows 350 cc.. this thread links the old gasgas tech page he used to manage..
did a bit of research myself after posting and 350-380 is just about right , should have guessed something was amiss as the manual says to check dip stick.
cheers Dave