If I recall, the lifters are retained in place to the black by a plastic retaining bracket, accessible only once the intake manifold is removed.
Are you sure it is engine oil in the coolant? Or could it possibly be automatic transmission fluid, indicating the heat exchanger in radiator has failed.
Changing the intake manifold is a bit of a pain, but can be removed in a couple hours while in the van. Just make sure you thoroughly clean everything before re assembly, including wiping it down with acetone , and use only The Right Stuff brand rtv, other cheaper brands will not stand up.
Again, removing the heads in place can be done, but it will be a pain getting everything out of the way, especially the rear exhaust manifiold.
How is the engine oil looking, any signs of coolant mixed with it?
Good luck
Jeff