If they're over-oiled, then yeah, I could see how they could cause a problem, but I've been running a K&N for years, with no issues, lightly oiled, both in a 3.8 and in a 4.6, with no problems
1997 T-Bird Sport, 'Interceptor' (12 ESP 2008-2012)- CAI, 2.25 duals, Flowmaster 40 series, Clear corners, BBK 70mm Throttle Body, TrickFlow upper intake, PI Intake, BBK adjustable fuel pressure regulator, LE stripes, Bullitt-style gauge faces. 18x9 wheels, Sniper tuned, 3,620 pounds
1991 Ford F250 - Air restrictor delete. Cat delete.
1991 T-Bird 'Interceptor II' (12 CP 2013-) - Race-prep, Full custom "interior", Carb'd 302, Trick Flow twisted wedge heads, stage 1 cam, Custom Koni coil-overs front, Koni yellows rear, Cobra front brakes, fuel cell, 18X9 wheels, 275/30 Hoosier A6, 4.11 T-Lok. 3000 pounds
2004 Toyota Solara SE - V6, Sport package, Muffler delete, 17X7 wheels, 235/50 Conti ExtremeContact DWS