As a car owner, I’ve often wondered, “What’s the thickest engine oil I can use safely in my vehicle?” Maybe you’re like me—trying to find the best oil for extreme heat, towing heavy loads, or dealing with an older engine that’s burning oil faster than it should.
In this article, I’ll take you through everything I’ve learned about thick engine oils, their applications, and when (or when not) to use them. Whether you’re trying to boost oil pressure, reduce oil leaks, or improve engine protection in severe conditions, this guide covers it all.
Understanding Oil Viscosity: What Does “Thick” Mean?
The “thickness” of engine oil is measured by viscosity—its resistance to flow. A higher number means a thicker oil.
For example:
| Oil Grade | Cold Start Rating (First Number) | Hot Viscosity (Second Number) |
|---|---|---|
| 5W-30 | Flows well in winter (5W) | Standard thickness at 100°C (30) |
| 20W-50 | Thick at cold temps (20W) | Very thick when hot (50) |
So, if we ask what is the thickest engine oil?, we’re looking for the highest viscosity rating, particularly the second number in multigrade oils.
What Is the Thickest Engine Oil?
The thickest commonly available engine oil is:
SAE 60 or SAE 70
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These are monograde oils, used in racing engines or vintage engines.
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SAE 70 is extremely thick—like molasses when cold.
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Not recommended for most modern engines.
Thickest Multigrade Oils:
| Oil Grade | Description |
|---|---|
| 25W-60 | Very thick in both cold and hot |
| 20W-50 | Common thick oil for older engines |
| 10W-60 | Used in high-performance engines |
Among these, 25W-60 and 20W-50 are widely used and available for commercial automotive use.
When Should You Use Thick Engine Oil?
Not all engines need or benefit from thicker oil. But in some situations, thick oil is exactly what your engine needs.
✅ Use thicker oil when:
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Your engine is old and burns oil
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You want higher oil pressure
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You’re in a very hot climate
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You tow heavy loads or do hard driving
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You drive a classic car or diesel truck that calls for it
❌ Avoid thicker oil if:
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Your car is newer and under warranty
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You live in a cold climate
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Your engine is designed for low-viscosity oils (e.g., 0W-20 or 5W-30)
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You want fuel efficiency
Pros and Cons of Using Thick Engine Oil
| Pros | Cons |
|---|---|
| Better protection at high temperatures | Harder to circulate in cold weather |
| Reduces engine wear in old/high-mileage cars | May reduce fuel efficiency |
| May reduce oil consumption and leaks | Can stress the oil pump |
| Better suited for high-load/towing use | Not ideal for modern small-clearance engines |
Can Using Thick Oil Damage My Engine?
Yes, if it’s too thick for your engine’s tolerances. Oil that’s too viscous can:
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Cause sluggish cold starts
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Starve internal parts of lubrication
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Overwork the oil pump
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Void manufacturer warranties
Always check your owner’s manual for recommended oil grades. Manufacturers design engines with specific oil flow requirements in mind.
Should I Switch to Thicker Oil if My Car Burns Oil?
Many people (myself included) have tried using 20W-50 or high-mileage formulas to reduce oil burning in older engines. It often helps. But remember:
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This is a temporary fix, not a repair.
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Thick oil may mask a deeper issue like bad valve seals or worn piston rings.
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Use it as a last resort or to extend the life of a beater car.
FAQ: Thickest Engine Oil
Q: What is the thickest motor oil I can buy?
A: SAE 70 is the thickest, but for regular use, 25W-60 or 20W-50 are the thickest practical multigrade options.
Q: Is 20W-50 oil thicker than 10W-40?
A: Yes. Both in cold and hot conditions, 20W-50 is thicker.
Q: Can I use thicker oil in summer?
A: Yes. In high temperatures, thicker oil can help maintain pressure and reduce wear.
Q: Will thicker oil stop engine knocking?
A: It may reduce it slightly in high-mileage engines, but it’s not a true fix for knocking, which is often mechanical or ignition-related.
Q: Is thicker oil better for high-mileage engines?
A: It can be. Many high-mileage oils are formulated slightly thicker to reduce leaks and oil burn.
Q: Can I mix thick and thin oils?
A: You can, but it’s not recommended. Stick to one proper grade. Mixing alters viscosity unpredictably.
Final Thoughts
If you’re dealing with an aging engine, extreme weather, or heavy loads, a thicker oil like 20W-50 or even 25W-60 might be your engine’s best friend. But for most modern engines, sticking with the manufacturer’s recommended grade is always the safest move.
