The thickness of engine oil, or its viscosity, is a key factor in protecting engine parts, maintaining lubrication, and ensuring reliable starting in various conditions. This article explains what counts as the thickest engine oil, how viscosity is measured, and when a high-viscosity oil might be appropriate or discouraged. It also covers practical considerations for modern engines, diesel engines, and vintage models, helping readers interpret labels like SAE and viscosity grades.
Understanding Viscosity And What “Thick” Means
Viscosity describes a fluid’s resistance to flow. In engines, oil with higher viscosity forms a thicker film between metal surfaces, reducing wear at high temperatures or under heavy loads. However, very thick oils can impair cold-start lubrication, increase parasitic power loss, and place extra stress on oil pumps. The Society of Automotive Engineers (SAE) assigns viscosity grades to lubricants, with “thicker” typically meaning a higher numeric grade at 100°C, or certain multi-grade combinations that perform well across a temperature range. For example, a 60 weight oil is thicker at operating temperature than a 40 weight oil.
Commonly Used High-Viscosity Engine Oils
Automotive engines generally use multi-grade oils that combine a low-temperature viscosity rating (the first number with a W, such as 20W) and a high-temperature rating (the second number, such as 50). The thickest popular consumer options include:
- SAE 60 — A single-grade oil used in some high-performance or older engines, and in certain motorcycles or off-road machines. It remains thick at operating temperature and provides robust film strength under high heat and load.
- SAE 50, SAE 60 in multi-grade blends — Some high-mileage or high-temperature environments use 15W-50, 20W-50, or 20W-60 blends to balance cold weather start-up with stronger film at temperature.
- 20W-60 and 15W-50 blends — Popular in high-performance, hot-climate, or older engines that tolerate higher viscosity to combat wear or leakage. These blends still allow a reasonable cold-start than a true single-grade 60, though they are thicker than typical modern standards.
For diesel engines, particularly in heavy-duty or high-load applications, high-viscosity oils such as SAE 60 and even some specialty diesel oils are common. In industrial or marine settings, oils with very high viscosity ratings (and even higher cSt values at 40°C or 100°C) are used to sustain lubrication under extreme conditions. Modern passenger cars rarely require such extreme viscosity, and using the manufacturer’s recommended grade is crucial for warranty and performance.
Why Some Engines Benefit From Thick Oils
Thicker oils provide stronger lubrication films at high temperatures, helping prevent metal-to-metal contact during long-haul driving, hot climates, or engines with higher bearing clearances. They can also offer improved protection in engines with rigorous tolerances or worn components where a thinner oil might thin out too quickly under heat. In vintage or high-performance builds, thicker oils can reduce scuffing and maintain pressure in older pumps. However, there is a trade-off: cold starts may be harder, fuel economy can decline, and the oil may not reach critical passages promptly in modern, tight-engine designs.
Limitations And Manufacturer Recommendations
Despite the appeal of very thick oils, modern engines are designed for specific viscosities. Using an oil that is too thick can starve the oil pump, delay lubrication during startup, and cause higher parasitic drag. Most manufacturers specify a viscosity range in the owner’s manual, often a multi-grade like 5W-30, 10W-40, or 0W-20. In hot climates, some engines tolerate 40, 50, or even 60-grade options, but only within the approved range. Always consult the vehicle’s specifications before choosing an especially thick oil, and consider using a high-viscosity option only when the manufacturer explicitly allows it or for engines that require it due to design or wear conditions.
Practical Guidance For Choosing The Thickest Suitable Oil
- Check the Owner’s Manual — The recommended viscosity range is the safest starting point for any engine.
- Consider Climate And Driving Style — Very hot climates or heavy-duty use may justify a higher end of the permitted range; cold climates generally require lower cold-start viscosities.
- Assess Engine Condition — Engines with worn bearings, high engine miles, or a history of oil consumption may benefit from a viscosity within the recommended range that offers improved film strength without sacrificing startup flow.
- Avoid Overly Thick Oil — Running a viscosity well above the manufacturer’s specification can reduce efficiency and cause startup issues.
- Follow Oil Change Intervals — High-viscosity oils still degrade with time and cycles; adhere to recommended service intervals to maintain protection.
Frequently Asked Questions
Is SAE 60 oil always the thickest option? In typical consumer automotive applications, SAE 60 is among the thickest common grades. Some diesel and industrial engine oils may push into higher viscosities, but they are often tailored to specific engines and conditions.
Can using a thicker oil damage my engine? Yes, if the oil is outside the manufacturer’s specification. It can impair startup lubrication, reduce pump efficiency, and lower fuel economy. Always prioritize the recommended viscosity for your engine.
Are there benefits to using thicker oil in cold weather? Thicker oil tends to flow more slowly at cold temperatures, which can hinder cold starts. Some engines benefit from a lubricant with good cold-flow properties (the first part of a multi-grade, like 0W or 5W). Stick to the recommended range for cold climates.
Conclusion
In practice, the thickest engine oil a vehicle should use is the highest viscosity within the manufacturer’s approved range. While SAE 60 and similar grades exist, modern engines are engineered around specific viscosity targets to balance protection, efficiency, and reliability. For most drivers, selecting an oil within the designed spectrum—accounting for climate, driving conditions, and engine wear history—delivers the best long-term performance. When in doubt, consult the vehicle’s manual or a trusted automotive professional to determine the appropriate high-viscosity option for a given engine.