Are Modern Engine Oils Getting Too Thin?
Are engine oils getting too thin? 5W-30 was once considered a relatively light engine oil, but today manufacturers specify 0W-20, 0W-16 and even lower-viscosity oils. The real question is whether engine protection has been sacrificed or whether engine and lubricant technology has simply changed.
Short answer: Lower viscosity does not automatically mean inadequate protection. Modern engines and modern lubricants have been developed together around lower-viscosity oils. But viscosity still matters. The correct oil must provide the viscosity, oil-film performance and specifications the engine was designed to use.
Look at engine oil recommendations over the past several decades and the trend is impossible to miss.
Oils such as 10W-30 and 5W-30 became common recommendations. Then 5W-20 and 0W-20 spread across the passenger-car market. Newer applications have moved farther toward grades such as 0W-16 and 0W-8.
That naturally raises a question: How thin can engine oil become before durability starts to suffer?
It sounds like a simple viscosity question. It isn't.
Engine design, bearing clearances, oil-pump strategy, operating temperature, base-oil quality, viscosity index, volatility, shear stability, additive chemistry and high-temperature oil-film performance all influence how a lubricant protects an engine.
Why Have Engine Oils Become Thinner?
The move toward lower-viscosity engine oils did not happen overnight. It has been developing for decades alongside improvements in engines, lubricant technology and fuel-economy requirements.
One important part of that development has been improved base oils. Higher-viscosity-index API Group II and Group III base stocks helped formulators produce lower-viscosity lubricants while controlling properties such as volatility.
Lower viscosity can also reduce the amount of energy an engine spends pumping oil and overcoming viscous drag. Reducing those losses can contribute to improved fuel economy.
Lubes'n'Greases recently raised the question of whether engine oils may now be approaching the practical lower limit of this decades-long viscosity reduction. The publication notes that lower-viscosity lubricants have played an important role in fuel economy while engine and transmission hardware evolved alongside them.
It also estimates that engine oil alone can contribute 3% or more to a fuel-economy index depending on the driving cycle.
The important point is that today's thin oils cannot be viewed independently from the engines designed around them.
Does Thinner Oil Mean Less Engine Protection?
Not automatically.
One of the biggest mistakes in engine-oil discussions is treating the SAE viscosity grade as if it were a complete measurement of lubricant quality.
It isn't.
The viscosity grade tells us important information about an oil's viscosity characteristics, but two oils carrying the same SAE grade can still differ considerably in formulation and performance.
Modern engine oils rely on a combination of base oils and additives designed to control wear, deposits, oxidation, corrosion, aeration, sludge and other problems while maintaining the required viscosity characteristics.
At the same time, there is a physical reason viscosity exists in the first place. Moving engine parts need a sufficiently strong lubricating film under their operating conditions.
So the correct conclusion isn't that thinner oil is always better or that thicker oil is always safer.
The engine and the lubricant have to work as a system.
Is 5W-30 a Thick Oil Now?
Here's the funny part: 5W-30 didn't get thicker.
The market moved around it.
When 10W-30 and heavier grades were common recommendations, 5W-30 represented the industry's movement toward easier cold-temperature operation and lower viscosity.
Now put 5W-30 beside 0W-20, 0W-16 and 0W-8 and it suddenly looks relatively heavy.
5W-30 hasn't changed its identity.
What changed is the range of viscosity grades surrounding it. That makes 5W-30 an interesting reference point for understanding how far modern engine-oil viscosity has moved.
5W-30 vs Today's Thinner Engine Oils
| SAE Grade | General Position | What It Shows |
|---|---|---|
| 10W-30 | Historically common multigrade | Shows where many passenger-car recommendations came from before widespread lower-temperature grades. |
| 5W-30 | Long-established modern grade | Once represented the move toward lighter oils and remains widely recognized today. |
| 5W-20 | Lower operating-viscosity grade | Became common as manufacturers pursued additional efficiency. |
| 0W-20 | Very common modern grade | Combines low-temperature performance with an SAE 20 operating-temperature grade. |
| 0W-16 | Lower-viscosity modern grade | Continues the move toward reducing lubricant-related energy losses. |
| 0W-8 | Ultra-low-viscosity grade | Demonstrates how far modern engine and lubricant engineering has progressed toward lower viscosity. |
This table should not be interpreted as a recommendation to substitute one viscosity for another. The correct viscosity depends on the engine, operating conditions and manufacturer requirements.
What About HTHS Viscosity?
This is where a discussion about "thin oil" becomes much more interesting than simply reading the number printed on the bottle.
HTHS means high-temperature high-shear viscosity. It evaluates viscosity under conditions intended to represent severe high-temperature, high-shear environments encountered in operating engines.
That's important because an engine doesn't spend its life sitting on a shelf at room temperature.
Oil operates hot and is forced through heavily loaded, rapidly moving components. Understanding protection therefore requires more than asking whether the bottle says 5W-30 or 0W-20.
HTHS deserves its own discussion because it helps explain why two oils that appear relatively close on an SAE viscosity chart may behave differently under demanding operating conditions.
Why Not Just Use Thicker Oil for More Protection?
Because more viscosity is not automatically more protection.
An engine's lubrication system is engineered around expected oil flow, viscosity, temperature and pressure characteristics. Changing to a thicker oil can change how quickly the lubricant circulates and how the lubrication system behaves.
Likewise, choosing an oil thinner than the manufacturer permits simply because thinner oils can improve efficiency ignores the requirements of the engine.
The practical rule: Start with the viscosity grades and specifications approved by the vehicle or engine manufacturer. If the manufacturer permits more than one viscosity for different temperatures or operating conditions, those recommendations provide the proper starting point for choosing between them.
Have We Reached Peak Thin Oil?
Maybe — and that's what makes this question worth watching.
Lubricant and powertrain engineers have spent decades reducing viscosity while maintaining the protection required by increasingly sophisticated engines.
But the benefit from lowering viscosity cannot necessarily continue forever.
At some point, the fuel-economy benefit from another viscosity reduction may become small enough that other engineering considerations become more important.
That does not mean 0W-8, 0W-16 or 0W-20 are "too thin." Engines designed and approved for those grades should be evaluated according to their actual lubricant requirements.
The more interesting industry question is whether future engines will continue pushing substantially below today's lowest viscosity grades, or whether engine-oil viscosity is approaching a practical floor.
Frequently Asked Questions
Are Engine Oils Getting Too Thin?
Not simply because they have a lower SAE viscosity grade. Modern lower-viscosity oils are developed for engines designed around specific lubrication requirements. Use the viscosity and performance specifications approved by the engine or vehicle manufacturer.
Does thicker engine oil protect better?
Not automatically. Protection depends on the engine design, operating conditions and the lubricant's complete performance characteristics. Using an oil thicker than required is not inherently an upgrade.
Why are manufacturers using thinner engine oil?
Lower-viscosity oils can reduce pumping and friction-related energy losses, contributing to improved fuel economy. Advances in base oils, additive technology and engine design have helped make lower viscosities practical.
Is 5W-30 considered thick oil?
5W-30 is not generally thought of as a heavy engine oil, but compared with newer grades such as 0W-20, 0W-16 and 0W-8 it now sits on the thicker side of many modern passenger-car recommendations.
Can I use 5W-30 instead of 0W-20?
Do not assume the two grades are interchangeable. Check the vehicle manufacturer's viscosity recommendations and required oil specifications. Some engines permit multiple viscosities under specified conditions while others have more specific requirements.
What does HTHS viscosity mean?
HTHS stands for high-temperature high-shear viscosity. It is a measurement used to evaluate lubricant viscosity under severe high-temperature and high-shear conditions relevant to operating engines.
The Bottom Line
Engine oil has unquestionably become thinner over the past several decades.
But saying "oil is thinner" doesn't tell us whether an engine is adequately protected.
Modern base oils, additive technology and engine design have allowed manufacturers to use viscosities that would once have seemed exceptionally light while pursuing better fuel economy.
5W-30 provides a useful reference point. It was part of the industry's movement toward lower viscosity, yet today it sits beside 0W-20, 0W-16 and 0W-8.
The question is no longer simply whether engine oil can become thinner.
It's how much farther engineers can reduce viscosity while still delivering the durability, protection and efficiency the engine requires.
Source: Lubes’n’Greases — Have Engine Oils Become Too Thin? , August 2026.