I took my F250 back to Ford for an
oil change and some warranty work.
I was told that if I changed the air filter in my truck to a K&N filter it would void all mechanical warranty on the truck. They told me they had a NEW F350 in the shop at that time (10,000 miles) (that I actually saw) that the diesel engine was getting replaced and it was "CAUSED DIRECTLY BY THE K&N FILTER" and would NOT be covered under any warranty.
I could not believe what I was hearing. The Ford rep repeated himself to make sure I heard correctly. The reason is they do not fit the F-series filter boxes correctly and they are less productive than the the factory filter that came in my truck from Ford. The filter lets unfiltered air into the engine at an enormous flow rate. They then took me out to the NEW F 350 and showed me that the K & N filter was the correct filter per K & N and that it simply did not fit the filter box.
I saw exactly what they were telling me. The filter did not fit the box well at all. It let unfiltered air into the engine from both sides.
I wrote the part number down and went to a local auto parts store and looked up the filter and part number for myself and the Filter in the truck at Ford did in fact have the correct part number in it. I asked the guy behind the counter and he told me the same thing that Ford was not standing behind engine failures for ANY reason if a K & N filter was in the truck.