Why do so many late model owners manuals still show : 3,750 mile OCI for severe duty and 7,500 mile OCI for non - severe duty for vehicles WITHOUT an OLM ? It would appear that everyone living in the suburbs (majority of people) qualify for severe duty usage according to the owners manual !! There appears to be a disconnect in modern vehicle owners manuals and oil chemistry advances . Now throw in a synthetic oil into the equation which should go 7,500 miles easy - but then there is the severe duty statement (anything and everything qualifies for severe duty - unless driving 55mph in Montana ). For those who drive vehicles without an OLM you either follow the owners manual or get a oil test at Blackstone - basically your only two choices ?