AMSOIL Signature Series 5W-40 100% Synthetic Max-Duty Diesel Oil
Developed to protect the horsepower diesel enthusiasts crave and extend the life of hard-working commercial equipment in the most severe conditions. Signature Series synthetic technology delivers max-duty protection in on- and off-road applications.
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AMSOIL Signature Series 5W-40 100% Synthetic Max-Duty Diesel Oil
AMSOIL Signature Series 5W-40 100% Synthetic Max-Duty Diesel Oil is a top-tier synthetic CK-4/CJ-4 motor oil for late-model emissions-compliant turbodiesel pickups, European diesel cars, and heavy-duty on- and off-road engines. The formula carries API CK-4/SN, CJ-4, CI-4+, CF, and CF-2, plus heavy-duty approvals from Volvo, Mack, Renault, Cummins, Caterpillar, Detroit Diesel, MB, MAN, MTU, JASO, Allison, Deutz, Scania, Chrysler, and Ford. Signature Series 5W-40 is the recommended Max-Duty grade for 6.7L Power Stroke, 6.7L Cummins, and 3.0L and 6.6L Duramax pickups, fleet trucks, and equipment that face cold starts, hard loads, and extended drain cycles.
Wear protection in heavy-duty service
Wear on rings and cylinder liners drives oil consumption up and pulls horsepower and fuel economy down. Signature Series Max-Duty delivers 6X more wear protection than required by the Detroit Diesel DD13 Scuffing Test for DFS 93K222, based on third-party testing using 5W-30 as a worst-case representation. Against a leading conventional 15W-40 in the same DD13 protocol, Signature Series Max-Duty kept cylinder liners looking like new. The 5W-40 records a 0.46 mm Four-Ball Wear scar (ASTM D4172).
Turbo cleanliness and tune-tolerant protection
Modern emissions-compliant diesels run higher EGT and tighter ring-land clearances, and aftermarket tuning compounds the load on the turbocharger. Signature Series Max-Duty delivers up to 60 percent better turbo cleanliness, based on specification standards of the CAT C13 2nd Ring Top Land Carbon test, and is built to withstand the extra demand placed on the turbo when pulling a load or running a horsepower tune. Sulfated ash of 0.99 weight percent keeps the formula compatible with DPF and SCR aftertreatment hardware.
Soot control and viscosity stability
Top-tier synthetic base oils resist oxidation and the thickening effects of soot contamination, while boosted detergent and dispersant additives keep soot particles suspended and independent. Suspended soot is less likely to agglomerate into the larger wear-causing particles that grind on liners, lifters, and rings. Signature Series 5W-40 holds an HTHS viscosity of 4.3 cP (ASTM D5481) and a NOACK volatility of 9.2 percent (ASTM D5800), retaining working viscosity through high-temperature service.
Cold-start flow and broad operating range
Signature Series Max-Duty resists thermal breakdown better than conventional and competing synthetic diesel oils. The formula contains no wax and stays fluid in sub-zero temperatures for easier starting and reduced cold-start wear. The 5W-40 grade reaches a CCS viscosity of 5,555 cP at -30 C (ASTM D5293), a pour point of -43 C (ASTM D97), and a viscosity index of 173, covering winter cold starts through summer heavy-towing service.
Reduced oil consumption and rust protection
Lower volatility means less evaporative oil consumption, and Signature Series Max-Duty delivers up to 76 percent less oil consumption than required by the API CK-4 standard in the Caterpillar 1N oil consumption test. The formula also provides up to 2X better rust protection in the NMMA FC-W rust test, important for equipment that sits seasonally or operates in humid and corrosive environments.
Recommended applications and service guidance
Use the live AMSOIL product page and your owner's manual to confirm the current viscosity, API category, OEM approvals, and service guidance for your engine before ordering. AMSOIL updates approvals and recommendations as formulas and manufacturer requirements change.
For extended service, follow the current AMSOIL guidance, your equipment manual, and oil-analysis results. Extended intervals are not appropriate for every engine or operating condition, especially modified engines, unusual fuels, racing use, or equipment with special service restrictions.
Typical technical properties
Property
Value
Kinematic Viscosity at 100 C, cSt (ASTM D445)
15.4
Kinematic Viscosity at 40 C, cSt (ASTM D445)
94.5
Viscosity Index (ASTM D2270)
173
CCS Viscosity, cP at -30 C (ASTM D5293)
5,555
Pour Point, C (ASTM D97)
-43
Four-Ball Wear Test scar, mm (ASTM D4172)
0.46
NOACK Volatility, percent weight loss (ASTM D5800)
Signature Series 5W-40 Max-Duty Diesel Oil is available in quart (DEOQT-EA), case of 12 quarts, gallon, 2.5 gallon, 55 gallon drum, and 275 gallon tote. Using AMSOIL synthetic lubricants and practicing extended drain intervals does not void a new vehicle or equipment manufacturer warranty, and all AMSOIL lubricants and filters are covered by the AMSOIL Limited Warranty. Match the right diesel grade and OEM approval set with the diesel motor oil products, match the right product in the AMSOIL vehicle lookup, then order AMSOIL Signature Series 5W-40 100% Synthetic Max-Duty Diesel Oil.
Customer reviews
4.9Based on 1,532 reviews
2016 F350 6.7 Powerstroke
I have used this since 2018. I follow the severe maintenance schedule for my truck due my driving style. It works great and I Love the convenience of having it sent to my house.
By Robert from St. Augustine, FL on May 13, 2026.
Would recommend this to a friend.
AMSOIL SIGNATURE SERIES
Expensive but cheaper than buying a new engine.
By Bill from Idaho on May 10, 2026.
Would recommend this to a friend.
All I use is Amsoil
Not much I need to say the product itself speaks for itself since I’ve switched to Amsoil I’ve seen a change in the way my engine runs and very impressed how the oil holds out
Is Signature Series Max-Duty 5W-40 the right oil for a 6.7L Power Stroke or 6.7L Cummins?
Late-model 6.7L Power Stroke and 6.7L Cummins pickups spec a CK-4 5W-40, and Signature Series Max-Duty 5W-40 (DEO) carries API CK-4/SN, CJ-4, CI-4+, CF, and CF-2 along with Cummins CES 20086 and CES 20081. Confirm the OEM viscosity and service category in the owner manual, then use the AMSOIL vehicle lookup to verify the fit.
Will this oil work in a Duramax LM2 or L5P?
The 3.0L LM2 and 6.6L L5P Duramax engines call for a low-SAPS CK-4 oil. Signature Series Max-Duty 5W-40 is a CK-4 formulation with 0.99 weight percent sulfated ash, compatible with diesel particulate filters and SCR aftertreatment. Follow the OEM viscosity recommendation for the climate.
Which is better in a turbodiesel pickup, 5W-40 or 15W-40?
A 5W-40 and a 15W-40 share the same SAE 40 high-temperature grade, but the 5W-40 flows like a 5W on cold starts. Most modern emissions-compliant pickup diesels spec 5W-40 from the factory because cold-flow performance reduces start-up wear and helps DPF-equipped engines reach operating temperature efficiently. Match the viscosity the owner manual prints and use the diesel motor oil products to compare grades.
How long can the oil stay in service?
Use the current AMSOIL product guidance, your owner's manual, and oil-analysis results to set the service interval. Extended service depends on the engine, operating conditions, fuel, filtration, and maintenance history.
Can Signature Series Max-Duty 5W-40 be mixed with conventional or other synthetic diesel oils?
The formula is compatible with other conventional and synthetic engine oils, but mixing shortens oil life and reduces performance benefits. Extended drain intervals are not supported when oils have been mixed, and aftermarket oil additives are not recommended for use with AMSOIL synthetic diesel oils.
Will running Signature Series Max-Duty void a new diesel engine warranty?
No. Using AMSOIL synthetic lubricants and practicing extended drain intervals does not void a new vehicle or equipment manufacturer warranty, and all AMSOIL lubricants and filters are covered by the AMSOIL Limited Warranty.
How do I confirm current approvals?
Confirm the current API category, OEM approvals, and viscosity guidance on the live AMSOIL product page before service. Approval lists can change when formulas or manufacturer requirements are updated.
