Re: 88 F150 oil in air filter
How many miles do you have on the engine and what kind of care has it had? Valve guide seals wear quickly on some early model engines and are not too expensive to replace. If it does need rings, the entire engine probably needs to be rebuilt and it would not be cost effective to just install new rings.
Here are a couple of things to consider. What kind of condition is the rest of the truck in, especially the transmission? If the truck is a "keeper" you can install a "short block" and have the heads rebuilt by a reliable local shop, or install a complete engine (heads and block). Don't forget to include the cost of installing the rebuilt engine, along with the cost of new belts, hoses and fluids.
I would also consider buying a good used newer truck before making a decision.