Providing you do not reach your preferred OCI mileage first (i.e. 5K , 7.5K , etc.) what is the max length of time (in months) you establish as part of your OCI routine with synthetic oils ? For most of us - it's usually a mix of highway and suburban driving with the occasional stop & go city trip thrown in here and there . Owners Manuals state either 6 months or 12 months max but of course there may be other variables such as whether you drive a GDI engine vehicle or not ? Since my main vehicle has a GDI engine , by 6 months the oil becomes fairly dirty (soot) and is starting to become a bit fragrant (fuel dilution) ... So , what is your max length of time you establish as part of your OCI routine with synthetic oils ?