Any recomendations on a good air filter replacement? I heard K&N are not so good, what about NAPA?

 

Nothing wrong with the specified Motorcraft air filter. I've used & changed them out as specified in my Ford scheduled maintenance call out, for the past 19 years in my 94 Taurus 3.8L & for 13 years in my 99 Ranger 4.0L & the intake ducting to both vehicles MAF sensor is still squeaky clean.
Scroll down to the 7th post on this thread to find the Motorcraft air & other filters preformance numbers. Motorcraft Filter Specifications - Bob Is The Oil Guy

Here is a link to some popular brand air filters for the Duramax diesel engine, tested & nicely graphed, to ISO 5011. The specified GM air filter did a bang up job!!!!
ISO 5011 Duramax Air Filter Test Report - MotoWiki

Some filter thoughts for pondering, let us know what you go with & how it does. Whatever you go with, make sure its seal area is good & that it fits the airbox seal rim right.

 

There's nothing wrong with a K&N if you're driving around in the aftermath of a volcanic eruption. Trouble is, they're designed as high-flow filters (WOT racing applications), not the best-filtering filters. Motorcraft is the brand to use.