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Synthetic Diesel Motor Oil FAQs

Diesel FAQs

No. Extending oil change intervals will not void new vehicle warranties. To affect the vehicle warranty, it must be determined that the lubricant was directly responsible for the failure; if the oil didn’t cause the problem, the warranty cannot be voided, regardless of brand or length of time in use. For additional information, visit, or contact the AMSOIL Technical Department at (715) 399-TECH (8324) or [email protected].

Yes. AMSOIL Premium 5W-40 Synthetic Diesel Oil, Premium 15W-40 Synthetic Diesel Oil, OE 15W-40 Synthetic Diesel Oil and OE 10W-30 Synthetic Diesel Oil are recommended for diesel applications calling for the API CJ-4 specification.

Yes. AMSOIL Premium 5W-40 Synthetic Diesel Oil, Premium 15W-40 Synthetic Diesel Oil, OE 15W-40 Synthetic Diesel Oil and OE 10W-30 Synthetic Diesel Oil are formulated with lower sulfur content to be compatible with 2007 and later high-speed, four-stroke diesel engines with exhaust after-treatment systems, and they also exceed previous diesel oil specifications. They are suitable for use in older diesel engines calling for API CI-4 Plus, CI-4, CH-4, CG-4 and CF-4 oils.

Each vehicle manufacturer defines "normal" and "severe" driving conditions for its equipment; AMSOIL recommends owners check these definitions in their owner’s manual to determine the service condition in which they typically drive.

AMSOIL defines severe service for diesel engines as extensive engine idling, daily short-trip driving less than 10 miles (16km) or frequent dusty-condition driving.

See the AMSOIL Product Recommendation and Drain Interval Chart for additional information.

No, you can use whichever brand of filter you choose and AMSOIL synthetic motor oils will still be guaranteed to protect your vehicle for the advertised period. However, you will need to change your oil filter according to the filter manufacturer’s recommendations and top off the oil.

Yes, it is normal for vehicles to use some motor oil. AMSOIL recommends checking your oil level every time you fill your fuel tank.

This is not true, Synthetic oil is perfectly safe for use in both new and high-mileage engines.

Synthetic oils do not cause engines to leak oil. In fact, AMSOIL synthetic motor oil is fully compatible with modern seal materials and is properly formulated to condition seals, keeping them pliable to prevent leakage. Early synthetics were made of esters, which were harder on seals, especially those made of neoprene. However, synthetic oils have come a long way since the early 1970s.

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 cold temperatures.

Yes. AMSOIL Diesel Injector Clean (ADF), Cetane Boost (ACB), Diesel Cold Flow (ACF), Diesel Injector Clean + Cold Flow (DFC) and Diesel Recovery Emergency Fuel Treatment (DRC) are compliant with the federal requirements for ULSD.

AMSOIL Diesel Cold Flow (ACF) is designed to prevent fuel from gelling and clogging fuel lines and filters. If the fuel is beginning to gel, the fuel should be treated with AMSOIL Diesel Recovery Emergency Fuel Treatment (DRC). Diesel Recovery disperses the wax crystals and allows the fuel to flow.

Yes. AMSOIL Engine and Transmission Flush (FLSH) is safe for use in diesel and gasoline engines and automatic transmissions to help restore equipment efficiency and operation. AMSOIL Engine and Transmission Flush is not recommended for manual transmission, CVT transmissions, differentials, transaxles, small engines or power sports equipment.

Elevated cetane levels, generally above 50, are preferable to get the most performance and power out of diesel engines. AMSOIL Cetane Boost (ACB) is engineered to improve the cetane value of diesel fuel up to seven points. AMSOIL Cetane Boost is appropriate for all types of service, including heavy- and light-duty service, on- and off-road and marine diesel engines. Cetane Boost is compatible with biodiesel and all types of exhaust emissions systems and is excellent for use with Ultra Low Sulfur Diesel Fuel (ULSD). AMSOIL Cetane Boost also improves operating cost by improving fuel economy, provides complete combustion for better power, reduces white smoke and emissions, and improves cold temperature starting.

Complete Diesel Protection
Diesel Truck