On my new exhaust system I have noticed the drive side tailpipe stays pretty clean while the passenger side one gets the black smuck build up....is this something I should be worried about...it seems in a days time (i do mostly highway drive-n) it builds up. Is this the sign of burn-n oil? if so what can I or should I do about it? Anything you can put in your oil to fix this? I now burn a synthetic blen..do I need to go full synthetic? Thanks for any help!