Why are API FA-4 diesel oils not backward-compatible? Answer: FA-4 oils feature lower viscosity grades and are designed to help next-generation diesel engines maximize fuel economy without sacrificing engine protection. These oils have limited backward-compatibility because some older engines were not designed to operate with lower viscosity grades. It's up to each engine OEM to determine if FA-4 oils can be used in its engines. Check the owner’s manual or with the OEM for specific recommendations.
AMSOIL Diesel Motor Oils
Signature Series Max-Duty Synthetic Diesel Oil 15W-40
Can diesel oils be used in gasoline engines? It depends; some AMSOIL synthetic diesel oils carry gasoline specifications. Check the product data bulletin for specific product and drain interval recommendations.
Can I switch from API CJ-4 or CI-4+ diesel oil to API CK-4 diesel oil? Yes, CK-4 oils are backwards-compatible, meaning they can be used in older diesel engines. CK-4 oils are designed as direct replacements for prior-generation oils, including CJ-4 and CI-4+ oils.
Does AMSOIL offer diesel oils for equipment calling for the API CK-4 specification? Yes. AMSOIL Signature Series Max-Duty Synthetic Diesel Oil and AMSOIL Heavy-Duty Synthetic Diesel Oil are recommended for diesel applications calling for the API CK-4 specification.
What is the difference between AMSOIL 5W-40 and 15W-40 diesel oil? AMSOIL 5W-40 and 15W-40 diesel oils are multi-grade oils, meaning they provide good performance in high and low temperatures. AMSOIL 5W-40 and 15W-40 both behave like SAE 40 oils in high temperatures; however, the 5W-40 is thinner than the 15W-40 at start-up and at cold temperatures.
