Much like every other forum there's a lot of questions but rarely actual answers. So I figured I should be part of the solution. I replaced the oil filter adapter gasket on a '06 GT in January 2012. It was leaking for weeks after an oil change at a dealer (doesn't matter which one - they all suck). I found the problem they didn't or didn't want to. The bitterness part ends here.
The gasket costs $15 at Fordparts dot com or $12 if buy it online and pick it up at a dealer. The dealer will charge you $15 if you buy it there. Make sure you get 6 quarts of 5W-20, anti-freeze, a new oil filter, and a tube of gasket sealant. The gasket might come with instructions, maybe. Get whatever you need to do a standard oil change.
- Start by emptying the oil pan and removing the oil filter.
- Replace (reinstall) the oil plug once all the oil has drained.
- Remove the hose near the oil filter adapter to drain the coolant. Don't drink the coolant! Move the hose out of the way once it empties.
- Remove the oil filter adapter via 4 bolts.
- Clean off the old gasket from the adapter and the engine block.
- Apply new gasket to gasket and use the gasket sealant to hold it in place.
- Align the oil filter adapter with gasket to the mounting holes on the block. Tight but don't damage the bolt heads.
- Reconnect the coolant hose.
- Install the new oil filter. Do not wiggle or shake it - the oil filter and attached adapter is NOT a rattle.
- Refill with new oil.
- Refill with new anti-freeze.
- Drop the car and crank 'er up! Move the car to clean ground and check for leaking oil or coolant. Check it again tomorrow! (Schedule it on your myPhone or whatever.)
If you don't fix this problem as soon as possible the bottom of your Mustang might look something like this (see below). I'm talking about oil all over the chassis and drivetrain, staining driveways, killing grass, cats and dogs sleeping together! End of the world kinda stuff. But even worse than that it can cause some damage to your engine (best observed by a clikkity-clakkity in some part of the engine bay you didn't know existed). I hope this helps.