One possibility is that you have blowby. I don't know how old your engine is or how many miles it has on it, but it sounds like excess pressure in the crankcase is blowing oil up through that tube into your air filter assembly. I know this can happen, as that is what my old engine did. Does the truck smoke? This is another indication of blowby.
Your carb could be oily just from the oil leaking down out of the air filter assembly (if that is possible). If it's gas leaking down, rebuild the carb.
I would do a compression check, if one or several of the cylinders are low, squirt some oil through the plug hole toward the cylinder; if the compression goes up, you have blowby.
If you have blowby, you will need to rebuild the engine eventually...
This is the worst case scenario. If you know the engine has low miles on it, or recently rebuilt, ignore this post.
Good luck!