Demand more from your oil. We do. The lineup of Mobil Delvac 1 heavy-duty diesel engine oils was designed to help commercial fleets drive business growth by boosting uptime and reducing fuel costs. Formulated to meet or exceed either API CK-4 or FA-4 requirements, Mobil Delvac 1 oils can help deliver fuel economy without sacrificing wear protection and can help extend engine life while extending oil drain intervals. Discover what our most advanced full synthetic oils can do for you.