Hi

I recently changed the plugs on my 06 x5 (x53) and noticed some oil on the first two chambers. The plugs towards the front also had oil on them. I cleaned the chambers and installed new plugs and noticed an increase in MPG and acceleration. Since then (3 months) the acceleration has decreased again and I'm worried the chambers have oil again. Is this is head gasket replacement or something else?

Thanks in advance for your help.