I don't know the proper name for the hose.... its supplies air to the passenger side valve cover, as part of the PCV system. If you have pressure in the crankcase, from a plugged PCV or worn rings, the pressure will blow the vapor up though the oil runback holes in the head, into the valve cover, and it then blows out the hole in the valve cover, into the hose and any entrained oil eventually ends up in the line, in the top of the throttle body, and oozing into the intake manifold. With enough vented vapor, the high velocity through the hole in the valve cover entrains the oil droplettes that the rocker arms are flinging around.

 

Many thanks for your help, I will take the inspection cover off the tb and look for oil, I think it sounds most definitely like i have a plugged PCV valve. Maybe I'll order a new one anyways, like you said 13 years is a little excessive!

Nice one!