Modern Engines Ask More From Engine Oil Than Ever Before

There is a common argument whenever the thin oils used in modern engines come up. The assumption is that manufacturers would really like to use thicker oil, but federal fuel-economy requirements force them to specify oils such as 0W20, 0W16 and, in some applications, even thinner viscosities.

There is some truth mixed into that argument. Lower-viscosity oil reduces internal friction and pumping losses, which helps fuel economy. Federal CAFE and emissions requirements give manufacturers plenty of incentive to find those efficiency gains.

The misconception is believing that fuel economy is the only reason these engines use thinner oil.

Modern engines are fundamentally different from the engines we were building 30 or 40 years ago. We haven't simply taken the same engine and poured thinner oil into it. Bearing designs, clearances, oil passages, oil pumps and increasingly complex engine-control systems have all changed along with the lubricant.

The biggest change is that engine oil is no longer being used only as a lubricant. In many modern engines, it is also a hydraulic fluid.

What Oil Has Always Done

The traditional jobs of engine oil haven't gone anywhere.

It needs to lubricate moving parts, maintain an oil film across bearing surfaces, reduce friction and carry heat away from areas where the cooling system doesn't have direct access.

The crankshaft rides on a hydrodynamic oil film in the main and rod bearings. The camshaft, lifters, rocker arms, piston skirts, rings and cylinder walls all depend on lubrication. Oil also absorbs heat from bearings, pistons, valvetrain components and other heavily loaded parts before carrying that heat elsewhere in the engine.

Older engines could certainly have hydraulic lifters and other oil-pressure-operated components. Using engine oil hydraulically isn't a completely new concept.

What has changed is how many systems depend on it, how precisely those systems need to operate and how closely their operation is monitored by the engine computer.

Engine Oil Has Become a Hydraulic Control Fluid

Look at what may depend on engine oil in a modern engine:

• Variable valve timing

• Variable valve lift

• Cylinder deactivation

• Hydraulic lash adjustment

• Timing-chain tensioners

• Multi-stage or variable-displacement oil pumps

• Oil-pressure-controlled valvetrain components

Not every engine uses every one of these systems, and the exact design varies considerably between manufacturers. The important point is that we're asking the same oil that lubricates the bearings to operate hydraulic controls.

That changes what viscosity means.

When oil's only job is lubrication, we can spend most of the discussion talking about film thickness, bearing clearance, temperature and load.

Once that oil is being routed through a solenoid and a small passage to move another component, we also have to care about hydraulic response.

Variable Valve Timing Is an Easy Example

Most hydraulic cam-phasing systems work by controlling oil flow into the cam phaser.

The ECM commands an oil-control solenoid, oil is directed to one side or the other of the phaser, and hydraulic pressure changes the relationship between the camshaft and crankshaft.

The ECM then watches the camshaft-position sensor to see whether the cam actually moved where it was commanded.

That means there is an expected relationship between the command, oil pressure, oil flow, temperature and how quickly the camshaft responds.

Viscosity is part of that relationship.

If the oil is substantially thicker than what the system was designed around, particularly when cold, it doesn't flow through an orifice or control valve exactly the same way. That can slow hydraulic response even though the oil is completely capable of maintaining a film between a crankshaft and bearing.

This is one of the areas where the old assumption that thicker oil automatically means better protection starts causing problems.

Pressure and Flow Are Not the Same Thing

A thicker oil will often give us a higher oil-pressure reading. That doesn't automatically mean we're getting better lubrication or better oil delivery everywhere in the engine.

Pressure is resistance to flow.

Increase viscosity and we generally increase resistance. That can raise the pressure we see while making it harder to move the same volume of oil through a small passage.

This becomes especially important during a cold start because all engine oils are much more viscous cold than they are at operating temperature.

If an engine was designed around a relatively light oil, putting a substantially heavier viscosity in it increases that difference even more during cold operation.

The bearings may have plenty of pressure while a hydraulic actuator farther downstream isn't receiving the flow or response rate the control system was designed around.

Cylinder Deactivation Is Another Hydraulic System

Cylinder-deactivation systems provide another good example.

Depending on the manufacturer and system, pressurized engine oil can be routed through control solenoids to special lifters or other valvetrain components. Oil pressure changes the mechanical operation of those components and allows the engine to deactivate and reactivate cylinders.

The oil isn't just lubricating the lifter anymore. It is participating in the operation of the lifter.

Now viscosity, contamination and oil condition become part of whether the system operates correctly.

This is also where small passages and control solenoids become important. It doesn't take a plugged main oil gallery to create a problem. A much smaller restriction can interfere with a hydraulic control circuit long before the engine experiences conventional oil starvation.

There Is Decades-Old Research Behind This

This isn't a new discovery in hydraulic systems.

In the early 1980s, the Fluid Power Research Center at Oklahoma State University conducted a study for the U.S. Army Mobility Equipment Research and Development Command examining contamination sensitivity in hydraulic cylinders, solenoid valves and servovalves.

The final report, FPRC-83-M-1, was published in 1984.

The researchers weren't testing automotive variable valve timing or cylinder deactivation. They were studying hydraulic equipment. That distinction matters.

What makes the research relevant today is what they were studying: what happens when contaminated fluid is asked to operate components containing valves, close clearances and precisely controlled hydraulic movement.

The work evaluated mechanisms including contaminant lock, silting force, wear and leakage and experimentally verified that hydraulic-component performance is sensitive to contamination.

That's a useful comparison when we look inside a modern engine.

We now have oil-control solenoids, spool-type valves, small passages and hydraulic actuators being supplied with the same oil that is collecting combustion byproducts and wear debris while lubricating the engine.

We didn't turn the engine into an industrial hydraulic system, but we did start applying many of the same hydraulic principles inside it.

Cleanliness Becomes More Important

This is where oil-change intervals become a bigger part of the conversation.

Dirty oil has always been bad for an engine. Contaminants accelerate wear, oxidation creates deposits and sludge can eventually restrict oil passages.

The difference is how much contamination it may take to create an operational problem.

On an older engine, we might think primarily about contamination severe enough to damage bearings, increase ring wear or plug a major oil passage.

A modern engine can have oil-control solenoids with small screens and passages, cam phasers, deactivation circuits and other hydraulic devices with much smaller working areas.

We don't necessarily have to reach catastrophic levels of sludge before contamination starts affecting something.

The FPRC research is useful here because hydraulic systems have dealt with this issue for decades. As clearances become smaller and controls become more precise, fluid cleanliness becomes increasingly important to component operation.

That's now something we have to consider with engine oil too.

We Can See It in Modern Engine Diagnostics

This isn't only theoretical.

Stellantis, for example, has published diagnostic guidance for the electro-hydraulic MultiAir system telling technicians to check for low oil, dirty or deteriorated oil, incorrect viscosity, oil contamination and a clogged oil-gallery screen before replacing the MultiAir actuator when diagnosing related trouble codes.

GM has similarly identified incorrect viscosity, dirty or contaminated oil, debris and aerated oil as possible causes of problems with its oil-operated Active Fuel Management lifters.

That doesn't mean every VVT, cylinder-deactivation or hydraulic-valvetrain code is caused by oil.

It means we can no longer separate engine-oil condition from engine-control diagnostics.

The computer can command a component to move and monitor whether it responds correctly. It doesn't necessarily know why that component didn't respond. A failed solenoid, mechanical damage, electrical fault, blocked passage, incorrect viscosity or contaminated oil can all potentially interfere with the result.

Modern Oil Pumps Add Another Variable

Oil pumps have changed as well.

A traditional fixed-displacement pump essentially produced more volume as engine speed increased, with the pressure-relief system controlling excess output.

That works, but pumping oil requires power.

Many modern engines use variable-pressure or variable-displacement strategies so the engine doesn't spend energy producing more oil pressure and volume than it needs at that moment.

At lower load, the engine may intentionally operate at an oil pressure that would make someone accustomed to an older engine nervous. When load and RPM increase, pump output can change.

Once again, the pump, bearing clearances, oil passages, viscosity and control strategy have been engineered as a system.

Looking only at the pressure gauge and deciding that more pressure must be better misses what the system is actually trying to accomplish.

CAFE Is Part of the Thin-Oil Story

Lower-viscosity oil absolutely contributes to fuel economy.

Less viscous drag means less energy is required to move the oil through the engine. Reducing that parasitic loss may only produce a relatively small improvement on one vehicle, but manufacturers operate under fleet-wide fuel-economy and emissions requirements. Small improvements across millions of vehicles matter.

EPA and NHTSA's own technical analysis treats lower-viscosity lubricants as a fuel-economy technology.

But that same technical analysis makes another important point that doesn't get nearly as much attention: moving to lower-viscosity oil can involve changes to crankshafts, connecting rods, main-bearing designs, materials and mechanical tolerances. It also specifically notes that lower-viscosity lubricants can improve the management of technologies such as cylinder deactivation and variable valve timing that depend on oil viscosity for operation.

That's an important distinction.

CAFE helped create an incentive to reduce viscosity, but manufacturers also changed the engines to operate with those oils.

Both things can be true at the same time.

Thin Oil Isn't Just an EPA Compromise

This is the misconception I think needs to go away.

A modern engine specifying 0W20 or 0W16 isn't necessarily an engine that would really be happier with 10W40 if fuel economy regulations disappeared tomorrow.

The engine may have been designed around the thinner oil.

Clearances can be designed around it. Pump output can be designed around it. Oil-passage dimensions can be designed around it. Cam-phaser response and hydraulic control strategies can be calibrated around it.

The thinner oil does reduce friction, which helps manufacturers meet efficiency requirements, but the engine is also taking advantage of its flow characteristics.

That's very different from saying thin oil exists only for CAFE.

Too Thick Can Create Its Own Problems

There are absolutely situations where a heavier viscosity makes sense.

If we significantly increase oil temperature, change bearing clearances, increase sustained load, raise RPM or introduce substantial fuel dilution, we may need to reevaluate the factory viscosity.

That's particularly true in performance and racing applications.

But that needs to be a decision based on what changed.

Putting significantly thicker oil into a stock modern engine because we assume thicker always means safer can interfere with hydraulic systems that were designed around a different viscosity.

A cam phaser can react differently. A hydraulic valvetrain system can respond differently. A cylinder-deactivation circuit can operate differently. A variable oil-pump system can see a different pressure-versus-flow relationship.

Move far enough outside the intended viscosity range and the computer may eventually recognize that something isn't responding as commanded.

That can mean a diagnostic trouble code rather than the additional protection we thought we were buying.

Oil Change Intervals Matter for More Than Bearing Protection

This is probably the biggest change in how I look at oil-change intervals on a modern engine.

We aren't simply asking, "Can this oil still lubricate the bearings?"

We also need to ask what condition the fluid is in while it is being used hydraulically.

As oil stays in service, it accumulates contamination. Fuel dilution can lower viscosity. Oxidation can increase viscosity and contribute to varnish and deposits. Additives are depleted. Combustion byproducts and wear material accumulate.

The FPRC research showed decades ago why fluid contamination matters when that fluid operates precision hydraulic components. Modern engines have added those kinds of components to the lubrication system.

That doesn't mean there is one universal mileage where every oil needs to be changed.

A vehicle that spends most of its life fully warmed up on the highway is operating under very different conditions from one that makes constant short trips, idles for long periods, experiences heavy fuel dilution or regularly sees track use.

The service interval needs to reflect how the vehicle is actually being used.

What has changed is the consequence of neglecting the fluid.

Dirty oil doesn't only have to remain good enough to keep two bearing surfaces separated anymore. It may also need to pass through a small screen, move through a control valve and accurately position part of the valvetrain.

The Oil Filter Is Part of the System

The same reasoning applies to the filter.

On a modern engine, the filter isn't only protecting the bearings from abrasive material.

It is helping protect oil-control solenoids, hydraulic valvetrain components, cam-phaser circuits, chain tensioners and other small oil passages.

Filtration efficiency matters, but so does flow capacity and bypass behavior.

The goal isn't simply to install the finest filter we can find. Just like viscosity, filtration has to work as part of the complete oil system.

The important point is that filter quality shouldn't be treated as an afterthought when increasingly complex hydraulic systems are sharing the engine's oil supply.

Modified Engines Make the Decision More Complicated

None of this means we can never deviate from the manufacturer's viscosity recommendation.

A modified engine may operate under conditions that weren't part of the original design target.

Road-course use can dramatically increase oil temperature. An engine build may change bearing clearances. Higher cylinder pressure can increase bearing loading. High RPM changes oil demand. Forced induction and some fuels can increase fuel dilution.

Those are all legitimate reasons to reevaluate viscosity.

But if the engine retains VVT, cylinder deactivation, variable oil pressure or other hydraulic controls, we need to consider those at the same time.

The question shouldn't be whether the factory viscosity is always right or whether thicker oil is always safer.

The question is what viscosity gives us the protection we need at the actual operating temperature while still allowing the rest of the oil system to function correctly.

Oil Temperature Gives Us Useful Information

For a performance application, actual oil temperature is one of the most useful pieces of information we can have.

Coolant temperature and oil temperature are not interchangeable.

An engine can maintain completely reasonable coolant temperature while oil temperature continues climbing during sustained track use.

As temperature increases, viscosity decreases.

If we're seeing oil temperatures well beyond what the stock vehicle normally experiences, moving to another viscosity may make sense. Now we're changing viscosity because we have identified an operating condition that requires it, rather than assuming a modified engine automatically needs thicker oil.

Oil temperature, bearing clearance, oil pressure, fuel dilution and the engine's hydraulic controls all need to be considered together.

Modern Engines Changed the Oil Conversation

Older engines already asked a lot from their oil. Lubrication, bearing protection and heat removal are major jobs.

Modern engines still require all of that, but we've added hydraulic control to the list.

The same oil can now be lubricating a crankshaft bearing while also operating a cam phaser, controlling a lifter, tensioning a timing chain and working with a variable-output oil pump.

That changes how we need to think about viscosity.

CAFE and emissions requirements have unquestionably encouraged manufacturers to pursue lower-viscosity lubricants because reducing friction improves efficiency. But reducing the entire discussion to "thin oil is just for CAFE" ignores how much the engine itself has changed.

Modern engines are designed around these oils, and many of their control systems depend on the pressure, flow, viscosity and cleanliness of the lubricant.

It also changes how we should think about maintenance. When engine oil doubles as a hydraulic fluid, keeping it within the correct viscosity range and keeping it clean become part of maintaining the engine-control system, not just the bearing surfaces.

That is why choosing the correct viscosity and changing the oil at an interval appropriate for how the vehicle is actually used matter more, not less, as engines become more complicated.