HTHS Viscosity
This article will help you understand base oil groups in the context of lubricants. We will explore their classifications ad main differences.
Table Of Contents
What Is HTHS Viscosity?
What is HTHS Viscosity?
HTHS Viscosity (High-Temperature, High-Shear Viscosity) is a fundamental and often misunderstood metric in the world of engine lubricants. It measures how oil behaves under extreme stress—specifically at and under high shear rates, mimicking the tight tolerances and high-speed bearing movement inside modern engines.
Unlike kinematic viscosity (which is measured at low shear), HTHS viscosity reflects real-world engine conditions:
- High RPM
- Narrow oil clearances
- Elevated operating temperatures
HTHS is expressed in centipoise (cP). A higher number generally means better film strength and wear protection, while a lower number suggests better fuel efficiency—but possibly less protection under heavy load if misapplied.
Why HTHS Viscosity Matters
HTHS viscosity is not just a technical detail; it is the single most critical measurement for predicting how an engine oil will perform under the most demanding conditions. It directly affects:
Wear protection under load:
Maintaining a strong, protective oil film to prevent metal-on-metal contact.
Fuel economy:
Lower HTHS viscosity oils reduce internal friction, leading to improved fuel efficiency.
Thermal and oxidative stability:
The ability of the oil to maintain its integrity under extreme heat.
Bearing durability and film retention:
Ensuring the oil film remains intact in high-pressure areas.
In short, it’s one of the most critical metrics for understanding how an oil will actually perform when pushed—far more useful than just looking at the label grade (like 5W-30).
HTHS vs. Kinematic Viscosity
While both measure oil thickness, they serve very different roles. Think of them as two different kinds of stress tests.
- Kinematic viscosity is a measure of how easily oil flows under gravity. It’s what you see on an oil label (e.g., 10W-30) and is measured at and .
- HTHS viscosity is a dynamic stress test. It tells you how the oil performs when squeezed between fast-moving metal parts under high heat and pressure.
That’s why a high-performance 5W-30 racing oil can perform far better than a 5W-30 economy oil—even though their kinematic viscosities are similar.
HTHS Standards and Limits
Different specifications have minimum HTHS thresholds to ensure performance. The move toward lower HTHS viscosity is a direct result of the automotive industry’s push for increased fuel efficiency and reduced emissions.
Low HTHS oils (2.6–2.9 cP) improve fuel economy but are not suitable for all engines. Always check your OEM requirements before using a low HTHS formulation.
Impact on Engine Wear and Fuel Economy
There’s always a trade-off between protection and efficiency.
Higher HTHS
- Stronger oil film
- Better wear protection
- Preferred for turbocharged, high-output, and older engines
- May slightly reduce fuel economy due to increased drag
Lower HTHS
- Less pumping loss
- Better fuel economy in EPA test cycles
- Often used in modern hybrids or GDI engines
- Risk of metal-to-metal contact under load if misapplied
A racing oil with 5W-30 viscosity may have an HTHS of 3.5+ cP, while an economy oil of the same grade might have just 2.6 cP. They will behave completely differently under stress.
Base Oils and HTHS Behavior
Base oil quality plays a big role in HTHS performance… Group III (hydrocracked): Better shear stability, but not as good as PAO. Group IV (PAO): Excellent shear stability, ideal for maintaining high HTHS
- Group I and II (Conventional): Tend to shear more under stress, lowering HTHS over time.
- Group III (Hydrocracked): Better shear stability, but not as good as PAO.
- Group IV (PAO): Excellent shear stability, ideal for maintaining high HTHS.
- Group V (Esters, etc.): Often used to boost film strength and polarity.
A synthetic PAO-based oil might maintain a 3.5 cP HTHS after 10,000 miles, while a Group II blend might drop from 3.2 to 2.7, leaving your engine with less protection than intended.
See our Base Oil Groups Explained guide for more on Groups I–V.
Synthetic vs. Conventional: HTHS Considerations
Synthetic oils typically offer:
- More stable HTHS across service intervals.
- Less viscosity loss under mechanical shear.
- Consistent protection at extreme temperatures.
Conventional oils can thin out more quickly, dropping below their original HTHS spec—a significant risk for heavy-duty or turbocharged engines. This is especially important in extended drain intervals or high-load use cases like towing or fleet driving.
Shear Stability Over Time
HTHS is not a fixed number—it can degrade during use. Over time, mechanical shearing inside the engine can thin out oil molecules and reduce the oil’s viscosity index, which can lower its protective properties. This is known as shear loss.
Factors influencing shear loss:
- Engine design (tight tolerances = more shear)
- Turbochargers (increased pressure and heat)
- Driving habits (frequent towing, racing)
Shear stability is the oil’s ability to retain its HTHS over time. High-end synthetics and performance oils often advertise “stay-in-grade” performance, meaning they hold their protective viscosity until the end of the oil change interval.
Oil Specs That Use HTHS
HTHS is part of the approval criteria for many key oil specifications:
- ACEA A/B/C sequences
- ILSAC GF-6 / GF-5 / GF-4
- API SN / SP Resource Conserving
- OEM specs like Dexos1, VW 504.00/507.00, BMW LL-01, etc.
It is also used in oil analysis and product marketing, especially when highlighting load-carrying ability and extended performance.
FAQ
What is a good HTHS viscosity?
It depends on your application. For most vehicles:
- 2.6–2.9 cP = Fuel economy formulations
- 3.0–3.5 cP = Balanced daily driver protection
- 3.5+ cP = High-performance, racing, or severe service
Always follow your owner’s manual or oil cap recommendation.
Can two 5W-30 oils have different HTHS?
Yes. HTHS varies by formulation even within the same grade. That’s why one 5W-30 might be better for towing or racing than another.
How do I find HTHS values for an oil?
Check the technical data sheet (TDS) or product datasheet from the manufacturer. HTHS is usually listed near kinematic viscosity.
Will low HTHS damage my engine?
Not if it meets your OEM spec. But if you use a low HTHS oil in a high-load engine (turbo, towing, fleet), it can lead to increased wear and reduced film strength.
Does HTHS affect oil pressure?
Indirectly. Thinner HTHS oils flow more easily, which may reduce oil pressure. But most modern engines are built for this. Just don’t mismatch specs.
How is HTHS measured?
HTHS is tested using specialized equipment like a tapered, rotating viscometer under ASTM D4683 or a similar method. These simulate engine-like conditions—high temp, high speed, tight clearances—to reflect what the oil experiences in real use.
Is higher HTHS always better?
Not necessarily. Higher HTHS oils may provide more protection, but they can reduce fuel economy. Balance your choice based on vehicle design, driving style, and spec requirements.
Can I mix high and low HTHS oils?
It’s not recommended. Mixing oils with different HTHS values can result in unpredictable protection. Stick to oils with matching specs for best results.