AMSOIL vs. Schaeffer's Oil: Which is the best diesel engine oil?

When deciding between premium synthetic oil brands like Schaeffer's and AMSOIL, it's important to look beyond marketing claims and examine actual performance data. As a former Ford automotive engineer with decades of hands-on experience testing and evaluating automotive systems, I've analyzed oils from both companies to help you make the most informed purchasing decision.

Brief Overview of AMSOIL & Schaeffer's Oil

Before diving into the technical specifications and performance comparisons, let's take a look at the background of both companies.

AMSOIL pioneered synthetic lubrication in North America when it was founded by Al Amatuzio in 1969 in Superior, Wisconsin. Originally named "Life-Lube, Inc.," AMSOIL introduced the first API-qualified synthetic motor oil in 1972 and has remained at the forefront of synthetic lubricant technology for over five decades, with an unwavering commitment to innovation and quality.

Schaeffer's Oil, headquartered in St. Louis, Missouri, has been in business since 1839, making it one of America's oldest lubricant manufacturers. The company began producing petroleum-based products in the 19th century and has evolved to offer synthetic lubricants for automotive, industrial, and agricultural applications. With over 180 years of history, Schaeffer's has built its reputation on providing specialized lubricants designed for severe-duty applications.

AMSOIL & Schaeffer's Similarities

Both companies specialize in premium synthetic lubricants designed for automotive, industrial, agricultural, and many more applications. They each offer a wide range of products including engine oils, transmission fluids, gear oils, greases, and specialty lubricants.

Both brands position themselves as premium options focused on providing superior protection and performance compared to conventional oils. They each emphasize extended drain intervals, though the specific recommendations vary between brands and product lines.

AMSOIL and Schaeffer's both maintain strong reputations among various niche markets—AMSOIL has historically been popular in powersports and racing applications, while Schaeffer's has built a loyal following in agricultural and heavy equipment sectors.

Additionally, both companies distribute their products through independent dealer networks rather than relying primarily on retail store distribution, though both brands have some retail presence as well. Now for the actual oil comparison.

AMSOIL vs Schaeffer's Oil Matchup

For a fair comparison, I'll evaluate similar product lines from both manufacturers. First, let's look at their maximum protection, flagship, fully-synthetic diesel engine oils: AMSOIL Signature Series Max-Duty Synthetic Diesel Oil 5W-40 and Schaeffer's 9000 SynShield OTR Plus Full Synthetic 5W-40.

AMSOIL Signature Series Max-Duty Diesel 5W-40 vs. Schaeffer's 9000 SYNSHIELD OTR Plus 5W-40

Let's breakdown the manufacturer-provided technical specifications for both oils:

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Based on the technical specifications I've compared between AMSOIL Signature Series Max-Duty Diesel 5W-40 and Schaeffer's SYNSHIELD OTR Plus 5W-40, here's my analysis of these premium diesel engine oils:

Key Performance Differences Between AMSOIL and Schaeffer's Diesel Oils

Viscosity Characteristics

AMSOIL Signature Series shows notably higher viscosity measurements at both 40°C (94.5 vs 85.50-89.00) and potentially at 100°C (15.4 vs Schaeffer's range of 13.50-15.00). This translates to a higher viscosity index for AMSOIL (173 vs 164), indicating better viscosity stability across temperature ranges. This is particularly important for diesel engines that operate under heavy loads and varying temperature conditions.

Cold Weather Performance

Both oils perform well in cold temperatures, but AMSOIL demonstrates slightly better cold cranking performance with a CCS viscosity of 5555 cP at -30°C compared to Schaeffer's 5913 cP. Lower CCS numbers indicate easier cold starts and better oil circulation during cold startups, which is crucial for reducing engine wear during the critical first moments of operation. AMSOIL Signature Series also shows a lower pour point in the technical specifications, further proving superior cold cranking oil flow and more protection at startup in cold climates.

High Temperature Stability

AMSOIL shows better high-temperature performance with:

• Higher HTHS viscosity (4.3 cP vs 3.91 cP), indicating stronger oil film strength under extreme heat and pressure
• Lower NOACK volatility (9.2% vs 10.7%), suggesting less oil consumption and better resistance to breakdown at high temperatures

Engine Protection Reserves

AMSOIL has a slightly higher Total Base Number (10.1 vs 9.74), providing somewhat better long-term acid neutralization capabilities. This can be especially important in extended drain interval applications or when using higher-sulfur diesel fuels.

Special Features

Schaeffer's highlights its use of proprietary friction modifiers including "Micron Moly®" and "Penetro®" that form a tenacious lubricant film on metal surfaces. They claim this provides better wear protection and increased fuel economy.

AMSOIL, meanwhile, focuses on its exceptional performance in standardized industry tests, including superior results in the NMMA FC-W Rust Test (though we don't have Schaeffer's data for direct comparison).

Service Life

AMSOIL confidently recommends up to 3X OEM oil change intervals for heavy-duty on-/off-road applications and 2X OEM intervals for turbodiesel pickups and gasoline engines compared to Schaeffer's vague verbage stating: "Longer drain intervals for lower overall maintenance costs." This significant difference could translate to fewer oil changes, reduced maintenance costs, and less downtime for fleet operators utilizing AMSOIL Signature Series.

Real-World Implications

For diesel truck owners and fleet managers, these technical differences translate to several practical considerations:

1. Extended Service Intervals: AMSOIL's longer and more concrete recommended drain intervals could reduce maintenance costs upwards of 50% compared to Schaeffer's.
2. Cold Climate Operation: AMSOIL's superior cold-temperature performance provides an edge for equipment operating in colder environments, especially for equipment stored outside or in unheated facilities.
3. High-Stress Applications: AMSOIL's higher HTHS viscosity and viscosity index suggest better protection for high-load, high-heat applications like heavy towing, mountain driving, or continuous high-output operations.
4. Oil Consumption: AMSOIL's lower NOACK volatility indicates less oil consumption between changes, which can be significant for engines with higher mileage or those operating continuously.

AMSOIL vs Schaeffer's Diesel Oil - Pricing

Another significant comparison to consider is the actual cost of both oils. Especially for fleet owners that have commercial businesses, it's great to know how much these oils will cost you in the long run. When comparing these premium diesel oils, it's essential to look beyond the initial purchase price and consider the total cost of ownership based on their drain interval recommendations.

When factoring in extended drain intervals, AMSOIL Signature Series Max-Duty Diesel 5W-40 offers better overall value than Schaeffer's 9000 SYNSHIELD OTR Plus 5W-40, especially with the Preferred Customer discount. Since Schaeffer's doesn't clearly recommend a concrete extended drain interval recommendation, we'll be fairly liberal and assume a 2X OEM interval for their oil. Even at that extended interval (3X vs. 2X for heavy-duty applications), factoring in the PC program discount results in approximately 26% lower annual oil costs for commercial trucks and 33% lower costs for turbodiesel pickups.

Additionally, AMSOIL's confidence in recommending these extended intervals without requiring oil analysis demonstrates their trust in their product performance and reduces the complexity and hidden costs of maintenance programs. The Preferred Customer program further enhances the value proposition if you're using multiple AMSOIL products across your vehicles or equipment.

*Note: AMSOIL doesn't recommend extended drain intervals for performance-modified engines, when using biofuels containing more than 10% ethanol or 15% biodiesel (B15) or for 2007-2009 Caterpillar C13 and C15 on-highway engines. Extended drain intervals are only recommended in these instances with oil analysis. 2007-2010 Doge, Ford, and GM turbodiesel pickups are also not recommended for extended drain intervals when using Signature Series Max-Duty Diesel 5W-30.

Conclusion - AMSOIL vs Schaeffer's Diesel Oil

While both oils are premium synthetic options that exceed standard API requirements for diesel engines, AMSOIL Signature Series Max-Duty Diesel 5W-40 demonstrates technical advantages in most measured specifications. This is reflected in their more confident and concrete recommendation of extended drain intervals.

For diesel engine owners seeking maximum protection and the longest possible service intervals, the data suggests AMSOIL has an edge. However, Schaeffer's product still offers excellent protection and includes unique friction modifiers that may provide benefits in certain applications.

The choice ultimately depends on specific operating conditions, maintenance priorities, and whether the longer service intervals offered by AMSOIL provide enough economic benefit to justify any price difference between the products.

Performance Testing - AMSOIL vs. Schaeffer's

AMSOIL published a performance test of the NMMA FC-W Rust Test comparing AMSOIL Signature Series Max-Duty Diesel with Lucas Magnum, Schaeffer's Durability Advantage, Shell Rotella T4 and Mobil 1 Delvac.

AMSOIL Signature Series Protects Against Rust

The test proved AMSOIL Signature Series 15W-40 Max-Duty Diesel provides up to 2X better rust protection than Schaeffer's 15W-40.

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Understanding the NMMA FC-W Rust Test

The NMMA (National Marine Manufacturers Association) FC-W Rust Test is a standardized industry test designed to evaluate an oil's ability to protect engine components against rust and corrosion. This is particularly important in marine environments and other applications where moisture contamination is a concern, but it's also relevant for all diesel engines.

During this test, steel test specimens are partially immersed in a mixture of the test oil and distilled water, then subjected to specific temperature and time conditions. The specimens are then evaluated for rust formation, with results measured in micrometers (μm) of rust coverage. Lower numbers indicate better rust protection, meaning the oil is more effective at preventing corrosion on metal surfaces.

For diesel engines, this test is significant because:

1. Water condensation is common in diesel engines, especially during cold starts
2. Extended idle periods can allow moisture to accumulate in the oil
3. Engines operating in high-humidity environments face increased rust risks
4. Rust and corrosion are leading causes of premature engine component failure

The AMSOIL test results showing a score of 17.25 μm versus Schaeffer's 39.5 μm demonstrates that AMSOIL provides more than twice the rust protection capability, which directly translates to better long-term protection for critical engine components.

Test results describe and represent properties of oils that were acquired on the dates listed in Table 1 (see www.amsoil.com/rusttest.aspx). Results do not apply to any subsequent reformulations of such oils or to new oils introduced after completion of testing. All oils were available to consumers at the time of purchase. Testing was completed in November 2017.

*Oils used: AMSOIL Signature Series 15W-40, Lucas 15W-40 Schaeffer's Durability Advantage 15W-40, Shell Rotella T4 15W-40, Mobil Delvac 1300 Super 15W-40, Reference oil (pass/fail limit) Max Cam Wear (µm)17.2533.6339.530.6336.6930.75

Is Schaeffer's Oil Any Good?

Schaeffer's Oil has built a solid reputation, particularly in the agricultural and heavy equipment sectors. Their products are generally well-formulated and provide good protection for most applications. Their longevity in the industry (since 1839) speaks to their ability to produce quality products that satisfy customer needs.

Where Schaeffer's tends to excel is in specialized applications like farm equipment and older diesel engines. Their formulations often include high levels of anti-wear additives and friction modifiers that can benefit these specific use cases. But AMSOIL also uses a lot of proprietary anti-wear additives and friction modifiers that are custom formulated for very specific applications like heavy, commercial equipment and high-mileage diesel engines.

However, based on the technical specifications and test results we've examined, Schaeffer's products generally don't match the overall performance level of AMSOIL in areas like thermal stability, cold-weather performance, and extended drain capabilities.

Why I Choose AMSOIL Over Schaeffer's

As a retired Ford automotive engineer who has spent decades working with engines and vehicle systems, I've seen firsthand the impact that oil quality has on engine longevity and performance. When it comes to my own vehicles and my recommendations to others, I consistently choose AMSOIL over Schaeffer's for several compelling reasons:

• ‍Clear Extended Drain Interval Guidelines: AMSOIL stands confidently behind their products with specific, measurable drain interval recommendations (3X OEM for heavy-duty applications, 2X for turbodiesel pickups). This transparency speaks volumes about their trust in their product performance. Schaeffer's more ambiguous approach to extended intervals suggests less confidence in their product's longevity.
• ‍Superior Cold Weather Protection: Living in Michigan, I've experienced how crucial cold-weather performance is for diesel engines. AMSOIL's lower cold-cranking viscosity and pour point have consistently provided better protection during those critical cold starts when most engine wear occurs.
• Lower Total Cost of Ownership: Despite a potentially higher initial purchase price, AMSOIL's longer drain intervals combined with their Preferred Customer program create a substantially lower total cost of ownership. For my own diesel trucks, I've calculated a 25-30% annual savings using AMSOIL compared to Schaeffer's—not insignificant when managing multiple vehicles or a fleet.
• Documented Performance Advantages: The technical specifications speak for themselves—from higher viscosity index to lower volatility and better rust protection, AMSOIL consistently outperforms Schaeffer's in standardized testing. As an engineer, I trust objective data above marketing claims.
• Personal Experience: In my F-Series Super Duty trucks, excavators, and other diesel equipment, I've consistently seen cleaner engines, better fuel economy, and more reliable performance with AMSOIL products. The reduced maintenance time and downtime have been invaluable for my operations.

My Recommendation

If you're looking for premium protection for your diesel engine along with the lowest total cost of ownership, I recommend becoming an AMSOIL Preferred Customer. You'll save up to 25% on all products, receive free shipping on orders over $100, and gain access to exclusive promotions and product specials.

Save 25% on AMSOIL Products

Frequently Asked Questions

Is synthetic oil worth the extra cost for diesel engines?

Absolutely. Diesel engines operate under higher compression, temperature, and often load than gasoline engines, making oil quality even more critical. Premium synthetic oils like AMSOIL provide better protection against the three main diesel engine killers: soot, heat, and acid. When you factor in the extended drain intervals possible with AMSOIL (up to 3X OEM recommendations), the synthetic oil actually costs less per mile while providing superior protection.

How does altitude affect diesel oil performance?

At higher altitudes, diesel engines typically produce more soot due to the lower oxygen concentration, which can challenge an oil's dispersant capabilities. AMSOIL's superior soot-handling ability makes it particularly well-suited for high-altitude operation. Additionally, the temperature swings often encountered in mountainous regions benefit from AMSOIL's higher viscosity index, which ensures consistent protection across a wider temperature range.

Can I switch from Schaeffer's to AMSOIL (or vice versa) without an engine flush?

Yes, you can switch between these premium synthetic oils without an engine flush in most cases. However, when switching from conventional oil or if your engine has significant sludge buildup, an engine flush may be beneficial. AMSOIL specifically states: "AMSOIL Signature Series Max-Duty Synthetic Diesel Oil is compatible with other conventional and synthetic engine oils. Mixing AMSOIL diesel oils with other oils, however, will shorten the oil's life expectancy and reduce the performance benefits."

Are these oils compatible with biodiesel blends?

Both AMSOIL and Schaeffer's oils are compatible with biodiesel blends, but with limitations. AMSOIL specifically notes that extended drain intervals are not recommended when using biofuels containing more than 15% biodiesel (B15). For any biodiesel blend above B15, I recommend following standard OEM drain intervals and monitoring oil condition with regular oil analysis.

How do these oils perform in emissions-equipped modern diesel engines?

Both AMSOIL and Schaeffer's oils are formulated to be compatible with modern emissions equipment, including Diesel Particulate Filters (DPF), Exhaust Gas Recirculation (EGR), and Selective Catalytic Reduction (SCR) systems. They meet the API CK-4 specification which is specifically designed for these systems. AMSOIL's lower volatility and ash content may provide a slight advantage for DPF longevity by reducing ash accumulation.

Do these oils affect warranty coverage on new vehicles?

No, both AMSOIL and Schaeffer's oils meet or exceed manufacturer specifications required for warranty coverage. Under the Magnuson-Moss Warranty Act, vehicle manufacturers cannot void your warranty simply for using aftermarket products. As long as the oil meets the appropriate API service classification and viscosity grade specified for your vehicle, your warranty remains intact. Always keep receipts and maintenance records as documentation.

Sources

• AMSOIL Signature Series Max-Duty 5W-40 Full Synthetic Technical Data Sheet
• Schaeffer's 9000 SYNSHIELD OTR Plus SAE 5W-40 Full Synthetic Technical Data Sheet