It's probably fine... if it pumps a decent dribble at idle it's likely o.k. Half a tank? sounds a bit low, but is that with the oiler tuned up max? The oiler screw should only rotate about 90 degrees (I think). If it goes around and around, the stop is broken off and you may not be at max output.
To get to the oil pump you need to remove the clutch etc.. are you up for this?
Make sure you don't have a real thick bar oil. Try some 20wt engine oil just fro a test.