Have Engine Oils Become Too Thin?

In my opinion, as a long-term formulator and industry observer there are several factors indicating that further widespread lowering of engine oil viscosity may not happen. The primary factor concerns the law of diminishing returns. As new base stocks appeared on the market in commercial quantities in the 1990s, it was easy to move from SAE 15W-XX oils to 10W-XX oils and then to 5W-XX engine oils. Thinner oils combined with the latest additive technology not only improved fuel economy but also maintained excellent protection of the engine hardware. By 2010, there was enough volume of Group III oils to enable 0W-20 oils to emerge for the mass market. Not only did the base stocks enable thinner oils, but their oxidative stability also continued to improve, allowing for longer oil drains and retained fuel economy throughout the life of the drain. Industry began seeing papers looking at even thinner oils, 0W-16 down to 0W-8 and even discussions around 0W-4 with the focus on further fuel economy improvement. There were calls for the Society of Automotive Engineers to define and allow for these grades. While research showed further fuel economy improvements, it did not disclose the proof of performance that the engine hardware was still adequately protected. While the focus in industry has always been on FEI, some may have forgotten that the primary purpose of the lubricant is to protect the engine!

Fred Girshick, a formulation and field-testing expert, who recently retired from Infineum, said that “SAE J-300 members were skeptical of extremely low-viscosity engine oils being defined by industry without sufficient research and field testing to prove that they can properly protect engines.” Girshick noted that “many formulators felt SAE needed to define the viscosity parameters for lower grades so that the research could be accomplished. A line was drawn at 0W-8 and that lead to the current version of SAE J-300.”

Once lower viscosity oils were defined, these SAE grades were introduced although commercial applications were limited, and most OEMs stayed with 0W-20. Fast forward to today and we still see limited applications below 0W-20. ILASC allowed GF-6B for 0W-16, but it came with significant debate and a different certification symbol. JASO and API (but not ILSAC), allowed their specifications down to 0W-8, but commercial use is still rare globally for oils below 0W-20. OEMs and their power train engineers seem to find 0W-20 for most light duty engines optimal for both fuel economy from the lubricant and appropriate hardware protection. Vehicles adopting even lower viscosity oils appear to be limited to hybrids where the lubricant operates at lower temperatures but even here it is not widespread. Given that most FEI from hybrids does not come from the small ICE engine, should any engine protection be put at risk? Not a question I can answer, but more knowledgeable users are reluctant to use even thinner oils. This is especially noticeable with diesel engines where users have been very slow to adopt API FA-4 and OEMs have been slow to recommend it.

Signs that OEMs may be thinking about moving back to thicker oils come primarily from warranty issues and recalls regarding potential wear issues and shorter engine life for gasoline engines. We investigated recent recalls from General Motors, Stellantis and Nissan and two came with changes to the OEMs engine oil recommendation. It should be noted that sources indicated other OEMs whose engines may have seen mechanical wear that led to warranty issues but were less public and without a specific change to its engine oil recommendations. This article is also not intended to indict thin lubricants as the sole cause of concerns or that industry should proactively shift to thicker oils. I have witnessed many field and engine tests that show that low viscosity lubricants can protect both gasoline and diesel engines and that does not include testing the OEMs may have done to validate their power train systems.

The most public issue came when GM recalled close 600,000 vehicles in early 2025 equipped with the L87 6.2-liter V8 engine. This recall was initiated by the National Highway Traffic Safety Administration and was very public due to many consumer complaints regarding loss of power. 

Reports described a sudden loss of power without warning and noted that the failures involved “a bearing failure that may result in either engine seizure or breaching of the engine block by the connecting rod” (INOA-PE25001-10002.pdf). The cause of these failures was attributed to some mechanical defects during manufacturing. The remedy was to inspect the engines and if issues were found the engine was replaced and owners could continue using 0W-20 dexos1 approved lubricants. If engines were not replaced, the owners were told to switch to 0W-40 dexos and received a10-year, 150,000-mile extended engine warranty. No public comments about potential loss of fuel economy with the thicker oil!

Stellantis recalled vehicles with small 1.2 or 2.2L engines using API or ACEA SAE 0W-20 or 0W-30 engine oils. The OEM recommended the engine lubricants be replaced with 5W-30 engine oil. Less details are public concerning the specific failures, but the OEM stated engine oil compatibility caused a concern with engine performance and longevity. Nissan also recalled 642,000 rogue vehicles equipped with a three-cylinder, 1.5L turbo charged engine. The failures caused seized engine bearings and other issues but did not involve a change to its oil recommendation. These recalls were OEM-initiated recalls with less details that could be found compared to the GM engines.

What we do see is that issues impact both small and large engines and have cost OEMs billions of dollars in warranty costs. Some of the recalls suggested manufacturing tolerances were not good enough to support the lower viscosity. 

Jeff Hsu, who is now a lubricants and fuel consultant and was a Technology Manager for lubricants development for Pennzoil and Quaker State motor oils at Shell commented: “From a hardware viewpoint, there is significant field data to show that lubricants had significant safety factors when it came to protecting the hardware at higher 5W-30 viscosity. Field tests run by marketers and additive companies showed that oils can survive and protect engines well past recommended oil drain intervals by the OEMs. Oils including 0W-20 have been tested at two to threetimes the recommend drains in the most severe services such as Las Vegas Taxis or Houston police cruisers, which would see extreme heat and idle time.” 

This data also helped OEMs to increase oil drain intervals from as low as 3,000 miles, to 5,000 miles and 10,000 miles in less severe service. Premium or full synthetic oils were advertised as capable to go to15,000, 20,000, 25,000 or more before changing with the testing to back up claims. As viscosity went lower testing was also done for 0W-20 or lower, but not all engines or hardware type can be tested in all duty services. It should also be noted that testing has become more expensive and harder to accomplish in a timely manner. This has led to significantly less field testing than seen 10-20 years ago. 

Safety factors regarding wear protection are hard to calculate but are reduced as you go lower in viscosity. At the same time, industry has also lowered ZDDP content in additives to help protect emission systems whose effectiveness can be reduced by the phosphorus and sulfur seen in additives. ZDDP has long been proven as an additive that protects against wear on metal services and has decades of proven performance. As levels went down, performance did not get worse, but it did not improve either and industry has settled on a range of 600-800 parts per million phosphorus coming from the ZDDP. Despite many claiming to have supplemental or primary anti-wear components other than ZDDP including some that are ashless, we have seen little change here in commercial additive technology. 

Cost and proof of performance of these alternatives are huge barriers to replacing something that works and is relatively inexpensive. With demand for engine oils declining due to longer drains and electrified drive trains, future investments in lubricant technology get harder and harder to justify, especially changing to new components that are more costly to manufacture, and further fuel economy gains are limited at best.

Research may be enabled down to 0W-8 but is it worth the investment to ensure hardware protection for limited FE improvement from the oil? It is hard to justify these investments economically and to allocate limited technical resources given competing business priorities. Manufacturing is also not a perfect science and risks that can lead to significant warranty issues is a major OEM concern as well. 

Girshick stated “For large engines, longevity and uninterrupted operations-avoiding downtime, outweigh minor improvements in fuel economy.” He added, there is a balance to be achieved to optimize both FE and durability and that should be industry’s goal not reducing safety factors so much that any manufacturing issues can lead to catastrophic wear.”

Hsu noted “OEMs no longer want to trade off protection for fuel economy as their reputations are on the line and the economic consequences are unacceptable. Many high-performance cars recommend 0W-40, why? Forged pistons and turbo charge engines want thicker viscosities (5W-30/0W-40) and consumers expect that the engine will not have any problems given the high cost of vehicles today.”

Today’s smaller engines deliver more Horsepower than ever and operate at very high temperatures, mechanical shearing and fuel dilution cause oils to see further thinning. Over time they may also thicken, but by that time, they have lost their oxidative stability and ability to deliver fuel economy. Oils should be changed well before they get to this point. Hsu stated “Once wear begins the oil can only prevent additional problems. Lubricants do not repair any damage that has occurred.”

It is hard to have a one size fits all engine oil that works in large and small engines, hybrids and conventional ICE vehicles, turbocharged or not and optimized for one viscosity grade in all operating conditions and environments. While there is little doubt lubricant challenges can be met with today’s additives and base stocks, there needs to be a balance to ensure one maximizes fuel economy but maintains adequate engine protection with a reasonable safety factor. Commercial applications, the number of products needed to be maintained by service providers and overall cost of maintenance are crucial considerations for OEMs and their recommended lubricant requirements.

 At the end of the day, OEMs will decide what is the appropriate SAE viscosity grade that should be used. For gasoline engines it’s not likely we see a rush back to heavier viscosity grades in most vehicles and 0W-20 oils have seen significant commercial use for over 15 years without many issues. Still, powertrain engineers may also decide that for some engines an 0W-30 or 5W-30 is the better choice to ensure longer engine life and avoid warranty issues. 

For most light duty engines, I believe 0W-20 will remain the primary recommendation for most engines. There is little additional FEI benefit moving to lower viscosity grades. By getting to 0W-20 the improvements regarding fuel economy from the lubricant has been achieved and seems to offer the right balance of protection and FEI. On the diesel side of lubricants PC-12 will enable XW-20 for larger engines but it could be decades before we see significant volumes of oils below XW-30. It is likely that 10W-30 will be the most widely used on road viscosity grade in the not-too-distant future if it’s not already there. Other large engines especially in off-road applications, may still stick to 15W-40 despite evidence that 10W-30 can improve fuel economy and still protect the engine. 

For both engines and transmissions, the ability to optimize the fluid to the hardware can be best achieved when additive companies and lubricant manufacturers work closely with the engine and transmission builders in the initial design of the hardware. The lubricant industry should also take significant credit for the improved fuel economy and lower emissions over generations while ensuring that both engines and transmissions are well protected. Finally, this does not mean that further improvements to FEI and lower emissions is not achievable, just that the benefits from engine oils may have peaked. Most improvements over time came from the hardware itself and advances that are still in play. We just need to remember the most important job of the lubricant is to protect and enable the hardware to run at peak efficiency while ensuring durability throughout the life of the vehicle. 

Steve Haffner is president of SGH Consulting LLC. He has over 40 years of experience in the chemical industry, priamarily with Exxon Chemicals Paramins and Infineum USA. Contact him at sghaffn2015@gmail.com or 908-672-8012.