Arh well that's just great Pete,,,,thanks mate
No XR600 expert,,,,,yet
Dry sump,,,the oil is held in the frame to be cooled.
The way I see it,,,is that that screw is a check inspection point,,,to make sure that the twin rotor pumping system is functioning,,,
Ok,ok ok,,,the frame downtube oil reservoir is piped to the intake of one pump rotor,,,,that rotor pumps oil to the head etc,,,,to check that circuit, one simply cracks open the oil delivery banjo feed on the cyl head whilst idling,,,if if pisses out? then that side pump rotor and delivery is functioning.
Now the way I see it is that the second pump rotor is responsible for keeping that frame reservoir full by scavenging the sump oil and pumping it to the frame. Now if you undo that check screw and the oil is below the hole level,,,then all is well, that side of the pump is evacuating the sump oil and pumping it to the frame. if it is above? then the evacuation process is lazy or not working.
So with the right amount of oil added after change,,,,the correct level is below that screw and within in the high/low mark on the frame dipstick. That is after running the bike and then idling it vertical for five minutes.