As I was working through my problems discussed in the post "Idles rough, runs poorly at low RPM", I discovered that the inside of the upper part of the intake manifold behind the throttle body had a black, wet coating. It smelled kinda like oil. The truck goes through some oil - about a quart or so every 3000 miles - but I think it's leaking most of it along the back of the intake and valve covers - another project to undertake. It has 260,000 miles on it, but it does not smoke except for an occasional blue puff when it is started (worn valve guides and seats?). Could a slight antifreeze leak from the throttle body cause this? I don't know what a mixture of carbon and antifreeze smells like. Ever ran into anything like this?

 

What year?
I'll assume you still have the PCV still attached to the back of the upper intake. The PCV runs from the pass valve cover to the back of intake connected with a hose to the #8 intake port.

This has been known to burn pistons. Frod re-routed the later model trucks to have it connect in the center. If your motor is getting weak (blowby) you could be filling the intake with oil. The fix for the routing is to take the hose off the back of the intake and plug it. Run the hose from the PCV to the center vacuum tree, use a T if needed.

I would pull the PCV and run the engine and look for blowby. It could just be a bad PCV but I would re-rout it anyway.

Are the valve cover baffles still in place? If not this could be the problem too.