On cars with an oil cooler, the O-ring between the adapter and cooler can leak. The O-ring can be replaced if you can get a new one from Ford. Some posted have reported the part is no longer available.
Another option is to replace the oil cooler with a standard Romeo oil filter adapter. Be sure to replace the block to adapter gasket at the same time.
Note, the same adapter is used on all SOHC modular motors. Crown Vic, Grand Marquis, Town car. The replacement does not have to come from a Mustang.
Best to use the Romeo version. The Windsor version is shorter. It can be made to work but expect things like the oil pressure sender being in a different location.
2003 GT Convertible (sold)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.