The average age of cars and light trucks on United States roads in 2018 was 11.7 years, according to data from IHS Markit reported in the Auto Care Associations 2019 Digital Auto Care Factbook. Nearly half of the 272.3 million light vehicles are more than 11 years old-a number that has crept up from 36 percent a decade ago. This is despite a 5.7 percent scrappage rate, the highest since 2013. In the medium- and heavy-duty segment, many of the 8.2 million on-highway trucks are replaced within 10 years, but construction and farm equipment is used considerably longer.

There likely is no process that can guarantee back serviceability. But lubricant-related failures are unlikely with most modern lubricants, especially if end users select an oil that is the right viscosity grade for their engine and meets current American Petroleum Institute quality standards-and if they stick to the recommended drain interval.

Its important to consider how hardware has changed over the years. For example, older engines were designed for 3,000-mile drain intervals, not todays common 5,000-mile intervals and certainly not the 10,000-mile drains allowed for many new vehicles at the right driving conditions.

Gordon Farnsworth, who was chair of the ASTM Sequence V engine test surveillance panel for 25 years, noted that engines have gone from slider-follower to roller-follower type valvetrains. Older engines took much longer to warm up than they do today, impacting low-temperature wear. Engine technology continues to move forward, and engines underwent significant changes in the past 10 to 15 years to make them more efficient. They have lower oil consumption, blow-by and emissions, less internal friction and better thermodynamic efficiency-not to mention less free fuel to cause deposits, which made long drains possible today.

Consider that today a four-cylinder turbocharged engine puts out more horsepower than many V8 engines in the past. Issues like [low-speed pre-ignition] are also causing additive technology to change. This makes comparisons to legacy engines or developing new engines that can evaluate performance in much older engines increasingly hard, Farnsworth observed.

Not every market around the globe operates in the same fashion as North America, which has typically been the only market where new oils tend to fully replace old ones and where it is generally agreed that the new products are good for all cars on the road. In other markets, such as Western Europe and emerging markets, new lubricants are used in new engines and older quality levels remain available for the existing car parc.

Phasing Out Phosphorus

Concerns about back serviceability largely swirl around phosphorus levels, which come primarily from the antiwear additive zinc dialkyldithiophosphate, among other components. The first Independent Lubricant Standards and Advisory Committee passenger car motor oil specification, ILSAC GF-1, introduced a cap on phosphorus in engine oils in 1992 at 1,200 parts per million because some findings blamed the element for fouling emissions control technologies. The maximum allowable amount dropped to 1,000 ppm with GF-2 in 1996, then to 800 ppm for GF-4 in 2004. This limit has stood through the current GF-5 spec and will remain in place for GF-6.

Owners of classic vehicles became concerned that new oils with less phosphorus would not provide the wear protection needed for older engines, especially ones that had been rebuilt. A simple solution was to use heavy-duty engine oil, which contains higher levels of ZDDP.

ZDDP is a proven antiwear additive and corrosion inhibitor that has provided strong protection for decades. Even today, ILSAC passenger car engine oil specifications carry a minimum of 600 ppm for phosphorus. However, levels have also begun to drop for heavy-duty lubricants, starting with API CJ-4, which introduced a 1,200 ppm phosphorus maximum that has remained in place for API CK-4 and FA-4. Heavy-duty specifications do not have a phosphorus minimum.

Some major heavy-duty brands have also reduced phosphorus levels to meet Europes ACEA E6 standards, as well as to enable production of universal lubricants that meet both API S (passenger car) and C (heavy-duty) categories.

Two years ago, Ford put out a consumer advisory discouraging use of API CK-4 engine oils because of wear concerns. Only recently has it permitted use of FA-4 in a small percentage of its newest light-duty models, and CK-4 oils with at least 1,000 ppm phosphorus in the formulation.

From a technical viewpoint, reducing ZDDP does not automatically lead to poor engine wear. Other chemistries protect against wear and can supplement or perhaps replace ZDDP. The challenge is proving that wear protection for older engines.

Dropping Viscosity

Perhaps a more impactful trend is lower engine oil viscosity. Increasingly thinner oils are being formulated to capture more fuel economy, with SAE 0W-16 creeping deeper into the North American passenger car market this year and SAE 0W-8 showing up in Japan.

Heavy-duty oils are shifting to SAE 10W-30 or lighter. API FA-4 allows for diesel oils with lower high temperature high shear viscosity, again to capture fuel economy. FA-4 is the first heavy-duty API service category that is not back serviceable, mainly because of the oils lower viscosity.

The next generation of passenger car engine oils will also be split into two specifications, one of which will not be back serviceable. ILSAC GF-6A will govern legacy viscosity grades, while GF-6B will cover SAE 0W-16 and lighter grades. GF-6B oils will sport a new Starburst symbol to help consumers recognize that they should not be used in older vehicles. Consumer and installer education will continue to be paramount in order to ensure that these new oils dont find their way into older engines.

Oils and Engines Dying Out

One way of predicting back serviceability is to look at the reference oils used to determine base oil interchange and viscosity grade read across. Typically these reference oils represent older technology, and by examining the performance of an old reference oil in a new engine, the industry has a tie-back with an oil that worked in older engine tests.

But availability of older reference oils is decreasing as inventory shrinks and reproducing them becomes more difficult. The components used in the past have evolved and may no longer be available. In some cases, even the base stocks are unavailable. Reference oil sponsors may work with OEMs to make substitutions that are appropriate, but additive technology is changing, making this strategy more difficult.

Field testing is another way to gather data, but running new oil technology in todays significantly different engine designs does not automatically allow a tie-back to older engines. Finding old hardware to test is all but impossible, since rebuilding such engines and defining a proper field test would be difficult without incurring extreme cost and logistical challenges.

The most important parameter to consider for back serviceability is wear protection. Other key performance demands such as oxidation, deposit and sludge control and emissions system protection, as well as oil pumpability to ensure the engine starts in cold weather, are less critical to back serviceability.

By far, wear protection is the main area of concern for most and the one area experts will likely spend the most time evaluating, Farnsworth noted. It is intuitive that lowering viscosity and lowering ZDDP would adversely impact wear protection. New hardware is of course designed to operate with lower viscosity oils, and other supplemental additives may provide additional protection.

But how can protection be ensured for older engines? This has become increasingly difficult, as OEMs find it difficult to develop new tests that can measure wear and differentiate reference oils, Farnsworth continued. In the past, we also had high- and low-temperature wear tests, but going forward, the industry is struggling with the Sequence IVB [for low-temperature valvetrain wear and scuffing], which along with chain wear are the only wear tests planned for GF-6, the next ILSAC passenger car engine oil category.

The Sequence IVB test sees very low levels of wear and does not appear to be a straight replacement for the current Sequence IVA. The test may not adequately predict performance in older engines, Farnsworth warned.

And chain wear is a new parameter, not one that has been looked at before. Although older engines had chains, they were shorter and less of a concern than in modern engines, he pointed out.

Is having a minimum phosphorus level and educating consumers about using the right viscosity grade enough to protect older engines? Modern oils are far superior in protection against oxidation, deposits and sludge, as well as protecting emissions systems. Rust, seals and low-temperature wear protection can be evaluated with bench tests and the old Sequence VIII test, which looks at lead bearing corrosion and will be the only test from ILSAC GF-5 included in GF-6.

But during an Auto-Oil Advisory Panel meeting in June, it was suggested that the Sequence VIII was no longer needed for very-light-viscosity engine oils that will be covered by ILSAC GF-6B.

Heavy-duty Carries Back

In contrast, the heavy-duty side of the industry has maintained several older tests that help ensure protection of legacy engines. Once hardware for these tests is no longer available, the Engine Manufacturers Association may also struggle with back serviceability. For the time being, it appears that marketers and OEMs are content with back serviceability of current oils-except for Fords concern about lower ZDDP levels.

Like their light-duty counterparts, heavy-duty lubricants have significantly improved in terms of oxidation, deposit and soot-induced wear protection and viscosity increase. It should not be difficult to conclude that new oils will perform well in older applications, with the possible exception of lower phosphorus levels and wear protection. Bench tests should also continue to manage corrosion and seal compatibility concerns.

Both heavy- and light-duty OEMs seem to have drawn a line in the sand with API FA-4 and ILSAC GF-6B, as these service categories will not support back serviceability. As the oldest vehicles continue to hit the scrap yards, perhaps the risk associated with using new oils in old engines also inches out of mind.

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