Why Modern Engines Are Switching to Lower Viscosity Engine Oils
Behind the drop in the viscosity manufacturers recommend for engine oil lies one key but little-known parameter: HTHS viscosity, which governs how much energy the engine spends pumping oil through narrow clearances. It is the steady reduction of HTHS, not simply a fashion for "thinner" oils, that explains why modern engines are increasingly designed around grades as low as 0W-16 and 0W-8. The reason lies in the simultaneous evolution of engine design, fuel-economy regulations, and additive chemistry.
Modern oils haven't become less viscous because of lower quality, but thanks to advances in engine manufacturing and additive technology, as well as tightening fuel-consumption regulations.
Why more viscous oils were used in the past
Manufacturing tolerances for engine components (piston-to-cylinder clearance, crankshaft bearing clearances) used to be noticeably wider, so a more viscous oil was needed to reliably fill the gaps and maintain the oil film. With less precise machining, a thinner oil could not have sustained sufficient pressure in the oil system or a protective film across larger clearances.
What has changed structurally
| Change | How it affects viscosity requirements |
|---|---|
| Manufacturing precision | Tighter clearances allow the use of lower-viscosity oil without losing system pressure. |
| Turbocharging and direct injection | Raise the thermal demands on additives, but don't by themselves require higher viscosity. |
| Variable-output oil pumps and variable valve timing systems | Are calibrated for a specific oil pressure and viscosity set by the manufacturer. |
| Fuel-economy regulations | Directly encourage the shift toward lower operating-temperature viscosity. |
What HTHS viscosity is, and why it's the key parameter
The key parameter behind the shift to thinner oils is called HTHS (High Temperature High Shear): the oil's viscosity at 150°C under a shear load simulating real conditions inside bearings and along cylinder walls. The lower the HTHS, the less energy the engine spends pumping oil through narrow clearances, which directly improves fuel economy. It is this reduction in HTHS, not just the winter "0W" rating, that underlies the modern trend toward low-viscosity oils.
An important nuance: the minimum HTHS threshold in the SAE J300 standard is tied to the second number in the viscosity designation (after the dash), not to the winter "W" rating. That's why 0W-20 and 5W-20 share the same minimum HTHS threshold, while 0W-16 has a lower one regardless of its winter rating.
| Viscosity grade | Minimum HTHS viscosity (SAE J300) |
|---|---|
| 0W-20 / 5W-20 | 2.6 mPa·s |
| 0W-16 | 2.3 mPa·s |
| 0W-8 | about 1.7 mPa·s |
| 0W-30 / 5W-30 / 10W-30 | 2.9 mPa·s |
| 0W-40 / 5W-40 / 10W-40 | 2.9 mPa·s |
| 15W-40 and above, including 10W-60 | 3.7 mPa·s |
| ACEA C5 (passenger car segment) | 2.6 mPa·s and above |
| API FA-4 (diesel segment) | 2.9-3.2 mPa·s |
For comparison: the sport oil grade 10W-60, used in high-performance engines (such as BMW M-series units), has a minimum HTHS viscosity of 3.7 mPa·s, more than double that of a modern 0W-8. This is exactly why the ILSAC GF-6 standard is split into two parts: GF-6A keeps a higher HTHS threshold for common viscosities (5W-30, 0W-20), while GF-6B applies only to 0W-16 with a lower HTHS threshold specifically for additional fuel economy.
The role of fuel-economy regulations
The shift to low-viscosity oils is driven not only by engineering capability but also by government fuel-consumption and CO2 emissions regulations in effect in the US, the EU, and other regions. Since reducing internal engine friction is one of the most accessible engineering measures for meeting such regulations, manufacturers use thinner oil as one of the tools for reaching their target efficiency figures.
Why you can't just choose a thicker oil
An engine is designed around a specific viscosity from the development stage: bearing clearances, oil pump output, and the logic of variable valve timing systems are all calculated for a specific oil pressure and viscosity. An oil thicker than the manufacturer specifies doesn't fill clearances "more reliably", it creates excess resistance to flow, which raises fuel consumption and, in some cases, can disrupt hydraulic systems designed for a specific oil pressure. Follow the manufacturer's recommendations rather than the general logic of "thicker is safer".
Common misconceptions
A thicker oil protects the engine more reliably.
No, not if the engine isn't designed for that viscosity. Excess viscosity creates unnecessary resistance and can disrupt systems calibrated for a specific oil pressure, including variable valve timing.
Low viscosity always means weaker protection.
No. Protection is determined not only by viscosity but also by the additive package and the precision of the engine's manufacturing. Low-viscosity oils specified by the manufacturer for a particular engine provide full protection.
0W means the oil is "too thin" for normal use.
No. "0W" describes only the oil's behaviour on cold start, not its viscosity at the engine's operating temperature, which is governed by the second number (for example, 20 or 16 in 0W-20 and 0W-16).
Frequently asked questions
- Can I put 0W-16 in an engine designed for 5W-30?
- Not recommended. The ILSAC GF-6B category (0W-16) is not backward compatible with GF-6A, since it is built around a lower HTHS viscosity threshold specifically intended by the manufacturer for a particular engine.
- Why does 0W-8 have such a low HTHS viscosity compared with more viscous grades?
- The 0W-8 grade was developed specifically for maximum fuel economy in engines with very tight manufacturing tolerances, where such low viscosity still provides a sufficient protective film.
- Will switching to a low-viscosity oil affect fuel consumption?
- Yes, but only if the engine is specifically designed by the manufacturer for that viscosity: reduced internal resistance delivers measurable fuel savings only in that case.
- Are carmakers worldwide moving to ultra-low viscosity at the same pace?
- No, but the difference lies less in the specific viscosities themselves (5W-20, 0W-20, and 0W-16 are all used by both Japanese and European manufacturers) and more in the approval system: in the US and Japan, the shift to ultra-low viscosity is formalised as a separate ILSAC GF-6B category, while in Europe similar solutions are more often implemented through individual carmakers' own approvals layered on top of the ACEA standards.
Conclusion
Modern engines and modern oils evolve together: lower viscosity has become possible thanks to more precise manufacturing and well-designed additive packages, and necessary because of fuel-economy regulations. The right viscosity choice should follow not the general idea that "thicker is safer", but the car manufacturer's exact requirements, including the HTHS viscosity parameter.