I'll try. I think OCI can go by group. In the old days only group 1 was availble and used for most gassers, and the squeezers used much more expensive group 2. The group 1 is good for 2-3 k miles or 2-3 months, this hold true for the stuff today. The group 2 is ok for ~6 months/miles esp in diesels where "fuel" dilution is with a near lube oil. Throw is more/bigger filters and easy to go longer. Recently? (last 15-20) group 3 is more avialable and cheaper, it is also much better, and good for about a year with some provisos. The worst SM rated oil is group2 and maybe some/all 2+, and thus can be used longer than in days of old(or lead).This I think can expalin some of the trend toward longer OCI's. Actually extend now would mean 6+ months on dino and 1+ yrs. on "syn". I did leave out grp4/5 and that is because it's different now and in the past, and only a small % of us use them.