Fouling a plug on 1995 Ranger

I am having a problem with fouling a plug on my Ranger about every 6 months.About a year back I noticed that I was using a bit of oil.I have never noticed oil leaking at any time and the truck does not smoke at all.However it begin to run a bit rough so I checked all 6 plugs and sure enough one was pretty nasty so I replaced it and the truck returned to normal operation.Over the past 2 weeks I have noticed the rough idleling has returned and I have had to add a little oil again.I am using a gas additive every fill-up to help clean the injectors and such.Can anyone give me some further advice? I hate to have to dig into the motor but the truck is otherwise in good shape with about 91K miles and no other mechanical problems.

Possibly a bad valve seal on the affected cylinder.