AMSOIL vs Castrol: Which oil reigns supreme?

As a former Ford Motor Company engineer with decades of experience designing, engineering and testing powertrain and chassis components and systems I can state from my first hand experience, the importance of using an ultra high quality synthetic engine oil. In this motor oil comparison of AMSOIL vs Castrol, we'll examine both companies' offerings to help you make an informed decision for your vehicle.

Overview of AMSOIL & Castrol

AMSOIL and Castrol represent two distinct approaches to synthetic lubrication technology. AMSOIL pioneered synthetic motor oil in North America, introducing the first API-qualified synthetic motor oil in 1972. Founded by Al Amatuzio in Superior, Wisconsin, AMSOIL continues to operate as an independent, family-owned company focused almost exclusively on synthetic lubricant technology with applications stretching wide from small, 2-stroke lawn equipment to industrial manufacturing, aerospace, and everything in between.

Currently, Alan Amatuzio (the founder's son) serves as Chairman & CEO of AMSOIL, they're still headquartered in Superior, Wisconsin, and they've grown considerably in the past several years with the launch of AMSOIL Industrial, partnerships with Lingenfelter Performance Engineering and Banks Power, and acquisitions of Benz Oil and Aerospace Lubricants.

Castrol's history dates back to 1899, when Charles "Cheers" Wakefield founded the Wakefield Oil Company in London. The company became Castrol in 1909, named after castor oil, a key ingredient in their early lubricant formulas. Today, Castrol operates as a subsidiary of BP (British Petroleum) and produces both conventional and synthetic oils. In 2024, marking their 125th anniversary, Castrol unveiled a three-pronged strategy called "Onward, Upward, Forward" that involves advancing mobility solutions for greater efficiency, improving performance for industrial customers, and exploring new opportunities like data center cooling solutions and battery thermal management.

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AMSOIL & Castrol Similarities

Both companies offer extensive product lines covering automotive, powersports, industrial, and marine applications. They share commitments to research and development, though their approaches differ significantly. While Castrol leverages BP's massive corporate infrastructure and research facilities, AMSOIL is still family-owned and operated and maintains independent testing facilities focused specifically on synthetic lubricant technology.

In motorsports, both brands hold high-profile partnerships. Castrol has a long history in Formula 1, serving as a technical partner to multiple teams including McLaren, Williams, and currently Alpine F1. They're also heavily involved in other motorsports categories including MotoGP, World Rally Championship, and NASCAR.

AMSOIL's motorsport presence focuses more on off-road and powersports racing, with major partnerships in Championship Off-Road racing, AMSOIL Snocross, and the Sturgis Motorcycle Rally. They've also established technical partnerships with performance engineering companies like Lingenfelter Performance Engineering, where their lubricants are exclusively used in high-performance engine development and testing for aftermarket kits on various makes and models like C4, C6, & C8 Corvettes, Camaros, CTS-V's, Escalades, Chargers, Mustangs, and many more.

Both companies maintain OEM partnerships, though in different segments. Castrol has several factory-fill agreements with manufacturers like Ford, Volkswagen, and Jaguar Land Rover. AMSOIL focuses more on aftermarket and performance applications, developing specific products for modified vehicles, severe service conditions and boasts extended drain intervals for normal service conditions on most passenger vehicles.

It's safe to say AMSOIL and Castrol have both demonstrated longevity and innovation in the lubricants industry, adapting to changing markets and technologies while maintaining strong brand identities.

The Synthetic Oil Revolution

A pivotal moment in synthetic oil history occurred in the late 1990s when Castrol faced a lawsuit from Mobil over the definition of "synthetic" oil. Castrol had begun marketing oils made with highly refined Group III base oils as synthetic, while Mobil argued only Group IV (PAO) base oils qualified as truly synthetic. The court's ruling in Castrol's favor fundamentally changed the industry - establishing that Group III base oils could be marketed as synthetic due to their chemical transformation during manufacturing.

This decision shapes the synthetic oil market today, allowing oils to be labeled "synthetic" if they use Group III, IV, or V base oils in any combination. AMSOIL used to advertise their products with a PAO badge, indicating they primarily used Group IV base oils, while Castrol and many other manufacturers often utilized Group III base oils in their synthetic formulations. Nowadays, most base oil formulations from the major brands are proprietary. For more info on what base oils and additives AMSOIL is made of, read here.

Castrol vs AMSOIL Matchup

Both of these oil brands produce a wide variety of lubricants for many different applications, so we'll be directly comparing oils of similar viscosities based on manufacturer recommended oil products, specifications, and approvals.

AMSOIL Signature Series vs Castrol EDGE Extended Performance

In this first matchup, we'll take the best of the best full synthetics from both brands to see how they compare across technical specifications and approvals. Both AMSOIL Signature Series and Castrol Edge Extended Performance claim to last 25,000 miles between oil changes for normal service conditions. So we'll compare 5W-30 weights for each.

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Both oils are approved for these specs:

• API SP/SN PLUS
• ILSAC GF-6
• GM dexos1 Gen 3
• Ford WSS-M2C946-B1
• Ford WSS-M2C961-A1
• Chrysler MS-6395
• Honda HTO-06
• ACEA A5/B5

Specs unique to AMSOIL Signature Series:

• ACEA A1/B1
• Ford WSS-M2C946-A
• Ford WSS-M2C929-A
• ILSAC GF-5, GF-4

Specs unique to Castrol Edge Special Tec:

• GM 6094M (this was superseded by GM dexos1 Gen 3)
• Chrysler MS-13340

From the industry approvals outlined above, both oils meet many current industry standards for modern engines. AMSOIL Signature Series 5W-30 has a slightly broader backwards compatibility with older Ford specifications and ILSAC standards. Castrol Edge EP also carries an additional Chrysler approval that AMSOIL doesn't mention.

Next we'll compare technical data provided by both manufacturers.

Manufacturer Specified Ratings

*hover over each specification for more details.

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Interestingly enough, while gathering research for this article, I found that the website regards EDGE Extended Performance as "Castrol’s strongest oil and the most robust engine oil available... provides the highest level of wear protection... for a full 25,000 miles!" Even the labeling on their oil containers claim 25,000 miles between oil changes.

Yet, the product data sheet from their site for this oil (updated Oct 2022) claims only "20,000 miles guaranteed." So we'll stick with the product data sheet for that value.

When comparing the technical specs between Castrol's and AMSOIL's highest quality synthetic engine oils, AMSOIL Signature Series is the clear winner on paper over Castrol EDGE Extended Performance. Both oils have nearly identical Viscosity Index ratings, suggesting they both experience similarly low decreases in viscosity with rising temperatures. But, AMSOIL out-muscles Castrol in every other rating both manufacturers provide. There's a fairly significant difference between CCS Viscosity and pour point for both, indicating AMSOIL performs much better at low temps and extreme cold starts.

The higher flash point and better kinematic viscosity @ 100C also point to better high-temperature protection and less oil burn-off.

Disclaimer:
It's important to know that not all of these ratings equally factor into the quality of any given motor oil. If you want to learn more about how motor oil's are tested read on here. In my experience, the three most important ratings of the ones that we compared above are viscosity index, pour point, and Cold Cranking Viscosity (CCS). These three ratings somewhat effectively sum up an oil's volatility and protection potential over varying temperatures.

AMSOIL SS vs Castrol EDGE - Pricing

Over a year of normal driving (15,000 mile average), you'll only need one oil change using both oils. Castrol EDGE Extended Performance comes out just slightly cheaper than the Preferred Customer pricing of AMSOIL Signature Series. I find it a bit concerning that their website and the product listings across various online and in-store retailers state 25,000 miles between oil changes for Castrol EDGE EP, even though the product data sheet only calls for 20,000.

If you want the higher quality, full synthetic oil, and you're willing to pay just a few dollars more, I recommend going with AMSOIL Signature Series.

AMSOIL vs Castrol Testing & Reviews

Unfortunately, there haven't been many independent lab tests directly comparing products from both of these oil companies. But I have found several customer reviews from AMSOIL where vehicle owners experienced significant improvements after switching from AMSOIL.

I have a 2001 gmc sierra 2500hd with the lq4 6.0 vortec, it has a bigger cam, longtubes with x pipe, cat delete, 42 pound injectors and some other performance mods. The truck has 210,000 miles on it and runs very healthy. Ive always liked amsoil products, ive noticed i burn less oil on 2000 mile trips. Castrol was the worst burning 1qt every 500 miles, valvoline and mobil 1 were about 1qt every 1000 miles and amsoil signature was at a half qt at 1000-1200 miles all were 5w30s. The zrod i put about 5000 miles on and the biggest thing ive noticed was it has less blow by than other oils ive used, i have a catch can to determine that. With zrod i had maybe an 1/8th cup of oil in the catch can after 5000 miles, im used to a quater or half a cup. The only thing i wish amsoil had was a high zinc and phospherous oil that was something other than 10w30, 10w40 or 20w50. I would like to see more variety like a 5w30 high zinc oil for people like me in wyoming with 40 below winters and dont care about emissions. All in all Its a good American oil.  - Caine in Wyoming

The only real downside is the price, switching from Castrol Edge to think, I could immediately feel the engine running much smoother, though I would NEVER in a million years go 25K miles on ANY on any oil, though some people would say it's fine if you change the filter, I just wouldn't do it - Jim in Colorado

AMSOIL or Castrol?

Both Castrol and AMSOIL have proven track records in the synthetic motor oil industry, with each bringing unique strengths to the market. From my engineering experience, life-long personal experience with vehicles & powersports equipment, my 25 plus years of selling automotive lubricants and the detailed analysis above, here's what stands out to me:

• AMSOIL Signature Series clearly bests Castrol EDGE Extended Performance in these areas
  • Better cold-weather & startup protection (lower CCS viscosity & pour point)
  • Higher flash point indicating better high-temp stability
  • Higher kinematic viscosity at 100C for better protection at operating temperatures
  • Guaranteed 25,000-mile drain interval that's actually backed by product data sheets
• Castrol offers:
  • Competitive pricing at retail
  • Decent OEM partnerships and factory-fill agreements
  • Okay technical specs, though generally behind AMSOIL's metrics

For most drivers, the decision will likely come down to price & performance. For those prioritizing maximum engine protection and verified extended drain intervals, AMSOIL Signature Series is the clear choice.

In my professional opinion, AMSOIL Signature Series offers a better overall value. For the protection you demand and the performance you deserve, choose AMSOIL.

Sources

• Castrol EDGE Extended Performance 5W-30 Data Sheet
• AMSOIL Signature Series 5W-30 Data Sheet

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