Product Reviewed: Signature Series Max-Duty Synthetic CK-4 Diesel Oil 5W-40 (DEO)
Current Rating: 4.87 out of 5 stars
Votes & Reviews: 204 vote(s) and 332 user reviews.
Reviewed by Steve: 
I use amsoil in everything I own, from vintage snowmobiles to outboard motors to stihl concrete saws to my cummins to my jeep crd. I swear by your products and have switched a number of people over to it. It's cheap insurance especially when you plan on keeping your investments for a long time. It's the only oil I can count on
Reviewed by DWC521:
My 2016 F350 6.7L runs smoother on this oil
Reviewed by Hurst:
I notice a big difference in mileage and how the truck ran after changing the factory oil to Amsoil. The mileage is up over brand X that I replaced. I've used Amsoil in all my diesel trucks, no complaints!
Reviewed by mistercedric:
I have used this oil for many years in my 3500 GMC. A recent trip of 11000 km with the 25000 mile oil filter did not change the color of the oil it was still as if it came out of the container. Excellent starting in cold weather
Reviewed by Ivan:
Great oil at better than dealership prices.
Why wouldnt you?
Reviewed by 100 MPH:
I love this oil I rely on it with my lively hood. Great stuff.
Reviewed by Danny Boy:
I don't know if my 2013 Ram diesel is protected better by using Amsoil motor oil, but I trust Amsoil research. I believe what Amsoil claims. I can only say this about Amsoil. Only time will tell as the engine ages.
Reviewed by Tamworthsat:
I would buy this top grade again because I once bought the XL grade and notice a slight drop in mileage.
Reviewed by Mark:
I have been using Amzoil in my 2008 Dodge Ram 2500 since the first oil change. I currently have 147,000 miles on it and it doesn't burn a drop of oil, runs smooth and has a ton of power. I believe that using a quality oil with regular oil changes is the key. This stuff is not cheap! Neither is this truck! But to replace it is $68,000. I think the Amzoil is going to give me that quarter million mile mark.
Reviewed by -King D:
Perfect
Reviewed by Benny:
I work my diesel trucks hard. Pull heavy loads 100% of the time. When it's 0° and they start up with quicker oil pressure and less engine clatter, or when it's 95° out and I'm wide open throttle for miles up a long steep grade and my oil temps maintain a steady temperature, I feel more confident my engine will last longer with Amsoil.
Reviewed by 204man:
Cold start 2016 cummins. -23. No problem. Amazing in cold temps.
Reviewed by Frank:
I have been using Amsoil since 1979 and I believe in putting the Best Oil in all my Toys . I use it in my cars,garden tractor , lawn mower , whole house generator , power washer , and my 2 Corvettes ,and in my 2013 Kioti CK35 HST Diesel tractor . I guess you could call me an Amsoil Junkie 😎
Reviewed by Arthur:
I just changed the oil in my 2015 with Amsoil for the first time, because the truck came with 2 years of maintenance
when bought new, and if you didn't use it your just wasting money.i am a firm believer in Diesel injector clean, I'm on my second Duramax and have never had a fuel related problem. I'll use Amsoil forever.
Thanks for great products.
Reviewed by Clay:
08 Duramax 250k miles and still going strong. -30 here in winter, starts up with good oil pressure no waiting on the pressure gauge to pick up cold oil.
Reviewed by JERRY:
When Amsoil produced a oil meeting Chrysler's requirements (2015 Jeep GC 3.0 Diesel) I jumped on it as I have always had good experience with Amsoil oil.
Reviewed by Marc:
Only product I use. The BEST!
I use it with AMSOIL FF and bypass filters.
Reviewed by Rob:
Excellent product, my 2005 Cummins runs smoother than it ever has. I feel very confident pulling some of the heavy loads that I do, knowing that this oil will stand up to whatever I can put it through.
Reviewed by Mark:
Very pleased with the purchase. Regens intervals have increased by a significant margin, particularly in the city driving.
Reviewed by Jeff:
I've been using this in my 2004 Dodge Cummins diesel for 12 years and 120,000 miles and it still runs like it was new.
Reviewed by Douglas:
Best oil on the market ! My 2011 Ford F 250 6.7 deisel has 99.687 miles on it and uses no oil between changes !!!
Reviewed by Dk:
Truck not plugged in -40c slow crank still started. I believe 5-40 made the difference. Good oil
Reviewed by Novy:
Been using this oil for a while and like it. Cold weather starts a breeze!!!!!!
Reviewed by big boy:
could not think of a better oil. longevity, performance, and mileage difference is evident. recommend it for all diesel vehicles. No complaints.
Reviewed by Ihmachine:
I use amsoil in my 07 diesel truck, I have 7,000 miles on the oil change and still clean according to the lab test! I've used amsoil in different motors and I'm a believer, this oil is spectacular from my personal experience.
Reviewed by BWW:
Have been using this product in my 2011 ford diesel since day one. I put on about 14,000 miles a year and change my oil once a year. I have never had to add oil between oil changes. I'm amazed even at change time the oil is still so clear its hard to read the dip stick. Ive been trying to figure out something to put on the dip stick to see the oil easier.
Reviewed by Kevin:
I have been using AMSOIL Premium 5W-40 Synthetic Diesel Oil for the last five years and I have never had any issues starting when the temperature drops below -30C/-22F.
I switched to AMSOIL after my first winter because I was not happy with the cold weather starting performance of other full-synthetic diesel oils.
Reviewed by Master Bear:
Amsoil Premium 5W-40 Synthetic Diesel Oil is the best! I have used it on my diesel vehicles for years, and have been quite satisfied with its longevity and performance.
Reviewed by JP:
2011 F250 6.7L. Great oil with great customer service to back it.
Reviewed by Mike:
Excellent performance in my 6.7 power stroke
Reviewed by Ronald Glen:
Switched over my 6.0 Powerstroke to
Amsoil DEO. After some contemplation, I decided to go with Amsoil for few dollars more than Rotella. I am glad with the decision as the engine has become noticeably smoother. This indicates lower internal friction which will translate into longer engine life. I am already getting 50-70 miles more miles per tank of diesel, which justifies for the slight premium in the price.
Also it appears that I can go longer on the oil. I personally don't believe & was not planning on extended drain intervals as suggested by Amsoil. But with the way things are going, I feel comfortable going 50% more on the oil, which again justifies the higher cost of the oil.
I'll continue with Amsoil DEO, its working out to be lot less expensive than RT6.
Re: Much smoother than RT6
To overcome extended drain anxiety, consider doing used oil analysis 1000 miles before scheduled oil change.
You can confidently, continue with the oil if the report indicates all the levels to be within spec. Repeat the oil analysis again until the report recommends changing the oil.
This is the most informed and scientific way to establish oil drain intervals. We see conventional oils breaking down below spec before their scheduled interval, indicating accelerated engine wear (even with frequent and regular oil changes). Therefore, the condition of the oil should be the factor that call for an oil change. Replied by Tech Admin
Canada
United States
Great oil for your engine
Great oil for you engine can't go wrong with full synthetic. Other then manufacture recommendation on oil brand this is the only other brand I use. I'm still old school so 5k oil intervals is cheap insurance for a $67k truck. Oil itself is amazing compared to previous oil used at 5k miles drains Amsoil looks and feels great. Smoother engine idle and smoother engine operation at higher rpm range. I have a digital psi gauge installed and with Amsoil the engine oil psi has maintained same psi throughout the oil intervals the oil temperatures has dropped 8 degrees lower under same driving conditions. I know that's not a professional oil analysis but it's what I've got.