Shining A Light on the Black Art of Motor Oil Selection

Motor oil selection, particularly for performance and racing cars, seems to be a “black art.” Most people rely on manufacturer guidelines, recommendations and past experience, as well as brand preference. Ask a hundred different people about this topic, and you get a hundred different answers.

So, to shed some light on the black art of choosing the slippery stuff, NASA Speed News spoke with oil expert Stefan Braun, an application engineer for U.S. Operations of Ulm, Germany,-based lubricants manufacturer LIQUI MOLY. Our discussion focused on motor oil selection for club and endurance racing, with particular attention to best practices for pre-event preparation and ensuring reliability for older engines found in a NASA endurance car competing in TREC or WERC, or another series.

Pre-Event Preparation

In terms of pre-event prep, one of Braun’s key pieces of advice for club racers, particularly when it comes to keeping operating temperatures low, is to do an engine flush every so often. He suggests a product like the company’s Pro Line Engine Flush, which is designed to loosen and dissolve sludge, lacquer and contaminants within the engine and then suspend those contaminants in the oil prior to a change with fresh oil.

Typically, you add engine-flush products into the crankcase immediately before an oil change, and idle the car – not drive it – for 10 to 15 minutes for the product to work contaminants loose. An engine flush procedure can help clean out oil passages and remove varnish, reducing drag to help oil flow more easily at high and low temperatures, and offer the potential to clean oil-actuated switches and valves, including systems like BMW’s VANOS and other variable-valve-timing systems. Engine flushes can be particularly helpful for motors like the old BMW M20 in the author’s Spec E30, which is a high-mileage unit from a street car with an unknown service history.

More than Viscosity: Choosing Oil Properties to Match the Engine
Moving from engine flushing to changing the actual oil, it might seem obvious that choosing the right oil should be based on manufacturer or engine builder specifications. Each engine has an American Petroleum Institute rating recommendation, as well as a range of given viscosities that are recommended by the manufacturer – and a good place to start is the owner’s manual or with one’s mechanic, engine builder, or even an online selection tool.

Viscosity, however, is a relatively basic measure of an oil. Rating an oil at a certain viscosity is simply an indication of how well an oil is able to flow at cold start and at 100 degrees Celsius. Braun says that viscosity, as dictated according to the Society of Automotive Engineers regulations, is not a performance indicator – but instead is just a rating of the oil’s ability to flow at two different temperatures. He further suggests that if one really cares about their investment in a race engine, one should look further into an oil’s ability to protect that investment by examining the oil’s properties and the basics of their engine’s design.

Regarding mineral versus synthetic oil, Braun suggests weighing a few factors for one’s own car. For engines that were originally designed with mineral oils in mind, Braun suggests that they may still benefit more from continued use of the same type of oil. He explains that mineral-based oil is “tackier” than synthetics, and therefore will stay on a friction surface longer allowing for better lubrication and cooling. Seal materials are also important factors, as older seals may not always be compatible with synthetic oils, and using less-tacky synthetics may cause leaks in older engines as synthetic oil can flow more easily past seals that worked just fine with mineral oils.

Braun says that synthetic oils are better suited to more “modern” engines, with tight bearing clearances, and turbo- or supercharging, which contribute to higher oil temperatures. This is because synthetics are usually more temperature-resistant, and more resistant to breakdown from unburned fuel accumulating in the oil. Synthetics and mineral oils are generally tailored to a specific need, based on the additive package. LIQUI MOLY offers some 34 variations on synthetic oil and eight mineral-based oil formulations with broad vehicle coverage, including specific oils for classics, to racing applications and more. Braun notes that LIQUI MOLY has sponsored pro race teams using mineral and synthetic oils with good results.

According to Braun, choosing an oil by viscosity, whether mineral or synthetic, alone is not always the best judgement, especially when it comes to racing. Oils can be different depending on formulation, consisting of the “base stock” and an additive package designed for each particular oil and application. As previously mentioned, the SAE viscosity rating for a given oil remains a basic fact about an oil’s hot and cold flow properties, and an important consideration for racing is that oil temperatures are regularly higher than 100 degrees Celsius.

One specification worth reviewing is an oil’s High Tension High Shear rating, which provides a better indicator of an oil’s ability to resist shearing at a much more representative temperature of 150 degrees Celsius. HTHS will reflect an oil’s ability to protect at higher temperatures, and while under pressure, similar to how it might flow in the narrow gap between a crankshaft journal and bearing.

Other factors that affect how an oil works in a specific application, such as evaporation loss, flash point, pour point, total base number and more, can impact the end result at the track, and should be evaluated on an individual basis, most likely by an engine builder. Braun also says that he finds it harder to change people’s minds when it comes to racecars and engines – but the current thinking in the industry is more focused on the properties of the oil versus the viscosity index.

Given an acceptable viscosity rating and an appropriate additive package, Braun suggests that to a large degree, most engines will be fine using the same oil. However, further gains can be made by “tailoring” the oil to each specific engine to gain power and efficiency, as well as to extend its usable life. He cites numerous variables for those who want to get the most out of an engine, which he describes as “including but not limited to” bearing clearances, bearing material, pistons, rings, and cylinder walls, as well as oiling system and oil pump types. Each combination can benefit from a different oil formulation, and in those situations, it is best to consult one’s engine builder. Interestingly, Braun says the physical layout of the engine – inline four or six, V6 or V8 – is not as big a factor as the above-mentioned variables.

Today’s trend in road cars is tighter clearances, and the type of oil that works in an older engine may not work as effectively in a modern tighter-clearance engine. The additive packages found in current street-car oils are quite sophisticated and apparently outperform many of the old-school “race oils” that feature ZDDP (zinc) as an additive. Manufacturer approval rules the road, and almost all manufacturers, from GM, to BMW to Ford, Mercedes and Porsche, have their own specific oil standards, which are a fine choice for cars that see street and light track duty.

For HPDE and track days, particularly when a car is in its warranty period, manufacturer-approved oils are the way to go to ensure that the warranty isn’t negatively affected. Braun notes while manufacturer oil specifications are important, they represent a “minimum quality” requirement, and oils from LIQUI MOLY, as well as those from other reputable manufacturers, far exceed these requirements. HPDE or track-day use means that a vehicle is often pushed harder than normal, and will see higher-than-normal oil temperatures as a result, so he considers it a good practice to shorten the oil change intervals to avoid pushing the oil beyond its limits – essentially, “cheap insurance.”

If a car is out of warranty, Braun says it is entirely OK to use an oil designed for performance or racing when going to the track. However, he cautions that oils designed for racing are just that – racing oils – and are designed for nothing else. So, when using “race oil,” one should change the oil quite often to ensure that the engine stays clean on the inside, as well as to prevent oil breakdown. Race oils are designed to be changed frequently, versus oils that possess original equipment specifications, which are designed to go the full oil change interval specified by the manufacturer. For track-day drivers who don’t want to spend extra money for oil changes, Braun suggests using anti-friction additives for improved protection and reduced oil temperatures.

Advice for Racers

For racers, a majority of oil selection criteria should revolve around viscosity index. A higher viscosity index helps an oil keep stable at temperature, and as an example, Braun offered the fact that Turner Motorsport uses a commercial oil, a LIQUI MOLY 5W-40 – with a high viscosity index – in its IMSA BMW M6. While this is not a very thick oil, the Turner team does see oil temps of up to 300 degrees Fahrenheit, and has reported no oil-related issues. The team regularly sends oil samples sent out for testing and analysis, and Braun reports that in every case, they have come back perfectly.

In general, Braun suggests that oils should be higher viscosity if they are to be run for a long time. This is because oil will become at least as hot as the coolant, usually in the 200-220 degrees Fahrenheit range, and maybe even hotter. Heat soaking associated with road racing cars, particularly those driven for many hours, means that higher weight, higher viscosity index oil are the better choice. In extended running, the friction created between the cylinder wall and the piston skirts and rings is a major contributor to engine heat, along with heat from combustion traveling into, and through the piston tops and cylinder walls. Friction from the crankshaft is another key cause of heat accumulation, and the proper oil combined with a well-functioning oiling system will help carry that heat away.

For example, he specified a synthetic 10W-60 for the BMW M20 powering my Spec E30, for its greater ability to remain dense at higher temps and carry away more heat, as well as for better sealing around the piston rings at high rpm over the course of the occasional endurance race or an extra-hot session at Thunderhill.

On their own, oils definitely can help reduce heat accumulation via a strong oil film that remains in place on moving parts long enough to remove heat, while simultaneously lubricating those parts. Oil also must protect from low-speed pre-ignition, particularly in turbocharged and direct injection engines, which can see high cylinder pressures at low speed and high load. Higher cylinder pressures increase the risk of LSPI damage, which can range from audible engine knock to complete engine failure, inclusive of heavy piston damage. Using the wrong oil can increase the risk of damage from LSPI, and newer oil specifications provided by vehicle manufacturers are taking LSPI into account when it comes to formulation and additive packages.

In addition to oil’s own properties for controlling temperature and addressing issues such as LSPI, there are some mechanical remedies available for racers. Cooling the underside of the pistons with oil jets also can be an important precaution, particularly relating to LSPI and high-boost, high-pressure applications, assuming it is a class-legal modification. Braun also suggests that external oil coolers are easy add-ons and should always be used if oil temperatures rise above normal range. Finally, the use of a dry-sump oiling system can further reduce oil temperatures and improve the effectiveness of the overall oiling system, but dry sumps can be cost prohibitive and not class-legal.

Additives

Once clean inside, a fresh oil fill and a new filter are the next step for one’s engine. In many cases, Braun also will recommend that racers use a friction modifier like the company’s MOS2 Anti-Friction Engine Treatment for some additional high-RPM protection. Companies like LIQUI MOLY consider their anti-friction blends to be proprietary science, so it does not divulge much about its formulas. But the basic idea is that these additives coat friction surfaces and other areas of metal-on-metal contact to form a sacrificial layer of lubricating material – be it metallic or ceramic – that wears away instead of the engine parts themselves. In general, Braun says that any “solid” anti-friction additive (MOS2 or Ceramic, like the company’s CERA TEC provides extra protection in the event of oil loss, while at the same time reducing friction to free up power, or simply to reduce wear. CERA TEC accomplishes this with the help of ceramic particles that flow in a boundary layer within the oil, and also provides a “proprietary chemical treatment” of all friction surfaces, which “polishes those surfaces for additional friction reduction when compared to other similar additives.”

In the event that one’s motor is consuming oil internally or has seepage from areas such as the valve cover gasket, products like Motor Oil Saver are designed to chemically interact with parts to make them swell enough to reduce leakage and somewhat stanch the flow. These additives address parts like valve-stem seals, or small rubber gaskets. It is important to note that this means minor leaks – this additive technology is not designed to magically repair questionable oil cooler lines or anything else that might fail catastrophically on track, and in so doing could endanger one’s fellow racers. Furthermore, none of these products are “engine rebuild in a can” that will magically save a tired motor. Best case scenario, they’re going to stretch that rebuild timeline a bit, offer a bit more peace of mind, and keep a solid motor running longer.

In the End, Common Sense

While it might seem obvious, Braun suggests that regular oil changes with an oil matched by manufacturer specs and viscosity index are the most effective way to keep track and dual-purpose cars running happy. And there’s no need to go overboard on oil change intervals, either – he pointed out that the Turner BMW team runs the same oil for an entire weekend and sends a sample out for analysis after each event, just to be sure.

He recommends oil changes every two months for track day/HPDE cars, unless there’s an excessive amount of track time and heat during that period. In addition to the Turner example, LIQUI MOLY supports other pro road racing teams that use mineral-based oil and an additive, (20W-50 Touring Tech and CERA TEC) and change oil just once for each race weekend. They also report outstanding oil analysis results. So club racers should be OK to follow the “once per weekend” guideline for oil service using synthetics, and even mineral oils.

In the end, oil selection doesn’t need to be a black art. It just requires some common sense and a little research about what’s right for a given motor, to keep it running for the long haul.

Special thanks to Stefan Braun and Mike Crosby of LIQUI MOLY for their expertise and participation. For more information, check out the LIQUI MOLY website or call them at 1-800-MOLYOIL.