They are about $18 at Auto Zone and Ford says they will go for ever. I do mine every 30k when I change the oil. They are usually clean when I replace them so I'm probably throwing my money away. But its not that big a deal when your spending $100 for Mobil 1 AFT.
Jim
I checked mine when I put the sonnax in, 89,000 miles, I have no idea if it had been changed before I got it but the thing was clean so we stuck it back in. Ill change it when I get around to switching over to synthetic.