JK makes an excellent point. The OEM will catch particles that get through the pre units. I think adding a finer filter, whether it's before or after is a good thing. I did both so I could argue either side. Actually, I have a 30 to 4 to 1 micron order. It will be the most efficient in theory, however, the amout is almost impossible to quantify in real life. Since the theory said it should be better, I went with post for my finer (aka Mega). Putting the 30 before the OEM is also helping. The more you filter, the cleaner the fuel, the longer she lives.
02 2500HD LT Dmax/Alli CC 4x4 Lt Pewter, Tan in, Amsoil, transynd, Bilsteins, Amsoil Air Filt, JK 4\" Exht, RollnLock, Rhino Liner, Remote start, alarm, High Idle, SPA DG211, Pred, Mega, Fumoto, Remote oil bypass, XL Tranny cooler, XM, Cell kit