03' XR50R Oil flow in airbox

I have an 01 that had that same tube with oil in it. I don't have the manual and never did figure out what its purpose was or how the oil got in there. I can't imagine that much oil could make its way from the case vent tube and accumulate in the airbox like that. My only thought is that it is some sort of self-oiling system for the air filter. I could be way off but that's my best guess. Of course, I've removed all that with the addition of the big-bore kit, but I remember thinking, "What the heck is all this?!"