ISL not performing

Both NAPA filters looked OK when we took them off and nothing untold came out when we dumped their contents into a pan. They did have slightly different "o rings" or seals (the loose ones that come in the package), but otherwise looked pretty similar to the Fleetguard ones we installed. The guy helping me also knew they were made by Wix and thought they were usually of pretty good quality. I was a little suspicious when I purchased the Fleetguard filters and found out they were almost twice the price of the NAPA ones. Maybe you do get what you pay for. The other thing I wondered about was the quality of the diesel fuel the guy who changed them last winter used to fill or prime them. If it was old or dirty, perhaps he clogged the filters with it right at the install.