Whilst cleaning out my boot today, I noticed my exhaust tip is heavily soiled in black soot , from the inside to the outer edges of the exhaust tip. I remember it being clean before so this must have happened last few months. I changed my oil a few months back and switched from semi-synth 10/40 to fully synth 5/40, so I am wondering could it be the oil is burning off heavily being thinner? Or I do have the EML on as my secondary 02 sensor is faulty with the crappy cat converter on, so I'm wondering if that would cause richer fuel mixture? Any ideas? Still got my VC to do so will take plugs out then to have a a look at them.