This has been the subject of much discussion, but the quick answer is yes, many people use Mobil 1 in their diesels. The correct designations for diesels begin with "C" (as in compression), these include CI-4, CH-4, CG-4, CF-4 and CF. Most people run 5W40 in older diesel. The Mobil 1 5W40 was formally available as "Truck and SUV" but is now being repackaged as "Turbo Diesel Truck" oil. As a result 5W40 is a little hard to find at the moment.