Castrol’s range of diesel engine oils are formulated to deliver ... T6 10W-30 is a full synthetic heavy-duty engine oil. A full synthetic 10W-30 provides the benefits of a full synthetic engine ...

10W-30, 10W-40, and 20W-50. The wide coverage makes Quaker State's availability its strongest feature versus competitors lower on the list. Castrol is another huge name in the motor oil business.