Cause of ignition coil going bad?

My son has a 1998 for F150 V8 5.4. We have replaced the number 3 ignition coil twice now, and now the mechanic says we have to replace it again along with the number 1 coil. There is oil on the bottom of the coil we took out. He says this could be something with compression or valves, but not for sure, but bad and will cost a lot of money. Can anyone help explain this to me?