I just bought a 92 Flareside that has 180k miles on it. I have a couple of problems.

#1 I'm getting some type of yellow gunk in my air filter coming back from the engine. I've changed/flushed the oil and still seeing a good bit ending up in the air filter case.

#2 To the best of my knowledge the truck has dual gas tanks and both 17 gallon capacity. On my last tank of gas coming from the back tank I barely got 7 mpg. The truck is not stalling and not missing, but it's sucking the gas. I have replaced the pcv valve. I have noticed that the valves are clicking when I accelerate between the 20-30 mph section.

 

Sounds like water and oil mixed.

 

yellow milkshake is a symptom of coolant and oil mixing.
Take a look in your rad, i bet the cap is nasty. Also, pull your dipstick when the engine is hot, if you see milkshake in either place you have major problems. Coolant will eat your engine bearings FAST.

 

The way I read the post he didn't say the oil was milk shake The yellow Goo at air filter and cap is condensation. Couple or more causes, truck not running hot enough, short trips, clogged pcv system. Engine can't properly boil off water vapor.

Tank issue sounds like fuel return issue.

 

I agree with SDMartin65. High mile motor with some blowby mixed with condensation to make the yellow gunk.
I don't think there's a selector valve per sey on the EFI setups, if I remember correctly the returns are just a single line T'd off to either tank. But that's just from memory. I would make sure the front tank isn't filling.
Valve noise at a certain speed range? If you truck has a tach, watch the rpm range where this occurs. If its noise from the valves, I would think it would happen a few times as the shifts come, and the engine goes through the rpm range again. Btw, what engine is in your truck?