I'm posting this to others like me who are learning about their 5.0's...
I replaced the PCV filter in my '91 5.0 today. It turns out that it's not simply a screen, but a small filter element. It's about 3/4" round and 3/4" high. It is made of a thin metal cylinder with a fine wire mesh inside. On the top and bottom are coarse metal screens.
Mine was apparently the original (or very near). It was completely clogged and was a real bugger to get out. I had to put a blanket on my left fender so I could lay over the engine compartment and get my arm between the intake and the firewall. I wound up taking it out piece by piece. I used a rag soaked in carb cleaner and a screw driver to swap out the hole. The new one just drops right in.
The clogged filter was responsible for oil in my intake, IAC valve, throttle body, MAF sensor and the rubber hoses between them. The oil blows up through the intake tube between the oil fill and the throttle body. At a cost of $6 I would recommend the filter be replaced often, even though it's a bit of a PIA to get to.
I have a couple of pictures of the old and new filters if anyone is interested.
'73 Convertible, 351C 2V
'91 LX Convertible 5.0
Member: Valley Forge Mustang Club