I think this system is a little different, and gets it's oil from somewhere else to oil the rocker shaft. I will have to look harder to find something on it. Here is a excerpt from a article in the tech section.
Additionally, oiling problems were common, and added to the poor reputation of the North American 2.9. Many poorly maintained or high mileage engines exhibit serious valve train noise. This problem is due to a number of design faults. Valve train oil pressure was entirely dependent upon a supply fed through the two center cam bearings. Even slight bearing wear could cause complete loss of oil pressure to the hydraulic valve lifters, rocker shaft, and rockers. Excessive internal “bleeding” through the half-moon camshaft thrust plate also contributed to this, though this can be remedied by reinstalling the thrust plate in an upside-down position. This condition is sometimes mistaken for ticking fuel injectors. This condition isn’t a cause for alarm unless it does it very loudly or after long freeway trips. If it does have the oil pressure checked, it could be low.