Hey again, I have a 1995 f150 5.8. I was changing my oil and air filter and noticed that there is a small rectangular filter (that connects to what I believe to be a suction hose?) under my air filter. It's in the same housing, it must have just been so dirty that I never noticed it... :I It is covered in a black mud-film. Is it the pcv filter? Can I replace it? Clean it? What is it? And what is its purpose? Thanks guys!

 

That's the engine breather filter. Just ensures fresh air is entering the crank case. You can change it, they sell them at the parts store

 

Pcv system. Change that, check your pcv valve in the passenger rocker cover. It should rattle when shaken. Good idea to replace - go to ford for that (motorcraft).
If the PCV system doesn't work properly, you'll find you're burning more oil than you would be if it was working. You can also build up enough pressure to start pushing oil out the front and rear main seals.

 

x2 on motorcraft only on PCV

 

x3