The Ultimate Guide to the Thickest Car Engine Oils: When and Why You Might Need Them
For the vast majority of modern passenger vehicles, the thickest engine oils available—typically those with a high-temperature viscosity grade of SAE 60 or even SAE 70—are almost never the correct or recommended choice. Using an oil thicker than what your engine’s manufacturer specifies can lead to poor lubrication at startup, reduced fuel economy, increased engine wear, and in severe cases, catastrophic mechanical failure. This guide will explain engine oil viscosity in detail, explore the rare and specific circumstances where extremely thick oils are used, and provide a clear, practical framework for selecting the right oil for your vehicle to ensure optimal protection and performance.
The cornerstone of understanding engine oil thickness is the SAE (Society of Automotive Engineers) viscosity grading system, notably the "W" and the number that follows it. This is not a single measure but a representation of the oil's flow characteristics at different temperatures.
1. Decoding the SAE Viscosity Grade: It's Two Numbers in One
An oil labeled SAE 15W-50 has two defined viscosity properties:
- The "W" (Winter) Number (15W): This indicates the oil's low-temperature, cold-start viscosity. A lower "W" number means the oil flows more easily at cold temperatures. For instance, a 0W oil is thinner and flows more freely at -30°C than a 15W oil. This is critical for getting oil to critical engine parts quickly upon startup, which is when a significant portion of engine wear occurs.
- The High-Temperature Number (50): This indicates the oil's viscosity measured at 100°C (212°F), simulating a hot, running engine. A higher number here means the oil is "thicker" or more resistant to thinning out when hot. Common grades for modern cars are 20, 30, and 40. Grades of 50, 60, and 70 represent progressively thicker oils.
The key concept is that modern oils are multigrade. A 5W-30 oil behaves like a thin SAE 5W oil in the cold to protect at startup but maintains the protective thickness of an SAE 30 oil at normal operating temperatures. This is achieved through advanced base oils and viscosity index improvers (VIIs). The goal is always to use the lowest possible cold-start ("W") viscosity and the correct high-temperature viscosity specified for your engine's operating conditions.
2. What Constitutes the "Thickest" Oils?
In the realm of commercially available engine oils for automotive use, the thickest high-temperature grades are generally SAE 60 and SAE 70. These are almost exclusively found in certain performance and racing oils, or in vintage car specifications. It is crucial to distinguish these from:
- SAE 50: A thick oil, but still used in some high-performance street applications, air-cooled engines, or under severe service.
- SAE 60 & 70: These are exceptionally thick fluids. At operating temperature, they have a much higher resistance to flow than a common SAE 30 or 40 oil. Their film strength is enormous, but their ability to circulate quickly, especially in cold conditions or in engines with tight, modern oil passages, is severely limited.
3. The Critical Role of HTHS Viscosity
Beyond the simple SAE number, a more important metric for protection in a hot, stressed engine is HTHS (High-Temperature High-Shear) viscosity. This measures the oil's resistance to thinning under the extreme shear forces present between bearing journals, piston rings, and cylinder walls. An oil must maintain a minimum HTHS value to stay in grade. Thicker high-temperature SAE grades (like 50, 60) inherently have higher HTHS, which is why they are associated with extreme pressure protection. However, many modern lower-viscosity oils (like some 0W-20 or 5W-30) are engineered with robust additives and high-quality base stocks to meet stringent HTHS requirements for the engines they are designed for.
When Might an Extremely Thick Oil (SAE 60/70) Be Considered?
These are niche applications, and switching to such a thick oil should only be done with explicit purpose and understanding, never as a generic "fix" for an old engine.
1. Dedicated Drag Racing and Certain Motorsports Engines:
In a top-fuel dragster or a purpose-built race car that operates at phenomenally high power outputs and temperatures for very short, brutal durations, a straight SAE 70 racing oil might be used. These engines are rebuilt frequently, are not concerned with cold starts (they are warmed up externally), and prioritize absolute film strength under astronomical cylinder pressures above all else. This is a world apart from street driving.
2. Some Classic/Vintage Cars with Original Engine Specifications:
Cars manufactured in the 1960s, 1950s, or earlier often had fundamentally different engine designs. They featured:
- Large bearing clearances: Worn or originally designed with more space between moving parts.
- Lower operating oil pressures: The oil pump systems were less aggressive.
- Different metallurgy and sealing technology.
The owner's manual for a 1960s muscle car might legitimately call for an SAE 20W-50 or even a straight SAE 50 oil in hot climates. Using a modern 5W-30 in such an engine could lead to low oil pressure readings and perceived lack of protection, though even here, a quality vintage-specific multigrade is often better than a straight-weight.
3. Severe Operating Conditions in Older, Worn Engines (As a Temporary Measure):
This is the most common misapplication. If a high-mileage engine is worn (particularly bearing wear) and exhibits very low hot idle oil pressure, some mechanics may suggest moving to a thicker grade (e.g., from 5W-30 to 10W-40 or 20W-50) to buoy pressure. This is a band-aid, not a cure. It may quiet down lifters and raise the pressure gauge reading, but it does not fix the wear. It also increases the risk of starving parts at startup. The proper fix is an engine overhaul. This practice should be considered a last-resort, temporary measure for an aging vehicle, always checking that the thicker oil can still circulate properly.
The Significant Risks and Drawbacks of Using Oil That Is Too Thick
Putting an SAE 60 oil in a modern engine designed for 0W-20 is a recipe for problems. The risks are substantial and often outweigh any perceived benefits.
- Poor Cold-Start Protection: This is the greatest risk. When you start your engine, thick oil is slow to travel from the oil pan up to the valve train, camshafts, and turbocharger bearings. This creates a period of boundary lubrication, where metal surfaces rub together with only a minimal oil film, causing accelerated wear. Most engine wear happens at startup.
- Increased Fuel Consumption: Thicker oil creates more internal fluid friction (drag) within the engine. The oil pump must work harder to circulate it, and moving parts like pistons and crankshaft fight against the oil's resistance. This directly reduces engine efficiency and increases fuel use.
- Reduced Power Output: For the same reason as increased fuel consumption, the engine's horsepower and torque output are slightly diminished, as energy is wasted pumping and shearing the oil.
- Potential for Oil Starvation and Flow Issues: Modern engines have precisely sized oil galleries and passages designed for specific oil flow rates. An excessively thick oil may not flow through small passages, oil coolers, or variable valve timing (VVT) solenoids quickly enough, leading to hydraulic lag in VVT systems, poor cooling, or even localized oil starvation.
- Increased Operating Temperatures: Because the oil is harder to pump and creates more friction, it can actually lead to higher overall engine oil temperatures.
- Overworking the Oil Pump: The pump must generate extreme pressure to force viscous oil through the system. This can put undue stress on the pump drive gear, chain, or belt.
How to Correctly Choose Engine Oil: A Step-by-Step Practical Guide
Forget what's "thickest." Focus on what's "correct." Here is the definitive process.
1. The First and Last Authority: Your Vehicle Owner's Manual.
Open the manual to the lubrication or specifications section. The manufacturer spent millions of hours engineering your engine and testing oils. They will list one or more recommended SAE viscosity grades (e.g., "Use SAE 0W-20"). They will also list a performance specification, such as API SP or ILSAC GF-6 for gasoline engines, or API CK-4 for diesel. Your chosen oil must meet or exceed this specification. This is non-negotiable for warranty and proper operation.
2. Factor in Your Driving Environment and Habits.
The manual often includes viscosity recommendations for different temperature ranges. For example:
- Extreme Cold (-30°C/-22°F and below): A 0W-XX grade is essential.
- Severe Service: This includes frequent towing, hauling, prolonged idling, or constant short trips where the engine never fully warms up. In these cases, staying with the recommended grade but using a full synthetic oil with a robust additive package is more important than arbitrarily thickening the oil. Some manuals may suggest a step thicker (e.g., from 5W-30 to 10W-30) for sustained high-temperature, high-load operation.
3. Understand the Impact of Mileage and Engine Condition.
For a high-mileage engine (generally over 75,000-100,000 miles), consider High Mileage formulated oils. These are not necessarily thicker. They contain specific additives to:
- Condition and swell older seals to prevent leaks.
- Contain extra detergents to clean accumulated deposits.
- Include anti-wear agents to protect worn surfaces.
They often come in the factory-recommended viscosity. If you have a verified oil pressure issue, consult a trusted mechanic before jumping to a thicker grade.
4. Select the Right Oil Type: Conventional, Synthetic Blend, or Full Synthetic.
- Full Synthetic: Provides the best performance across all temperatures. It flows better at low temps and resists breakdown better at high temps. It is the best choice for most drivers, especially for extreme conditions, turbocharged engines, and extended drain intervals.
- Synthetic Blend: A mix of conventional and synthetic base oils. Offers better protection than conventional at a moderate price point.
- Conventional: Adequate for very mild driving in engines that specify it. Largely surpassed by synthetic technology.
Practical Oil Change Tips When Using Correct Viscosity
- Always replace the oil filter with a high-quality filter rated for your engine and oil change interval.
- Ensure the engine is warm (not scalding hot) before draining to suspend contaminants and improve flow.
- Use a torque wrench to install the drain plug and filter to the manufacturer's specification to prevent stripping or leaks.
- Check the oil level with the dipstick 5-10 minutes after shutting off a warm engine, on level ground. Keep it between the "MIN" and "MAX" marks. Overfilling can be as harmful as underfilling.
- Record your mileage and date to maintain a consistent service interval.
Addressing Common Myths and Misconceptions
- Myth: "Thicker oil always provides better protection."
Fact: Protection comes from maintaining a proper oil film, which requires the correct viscosity. Too thick fails to flow and protect; too thin fails to maintain film strength. The right balance is key. - Myth: "If my engine is burning oil, I should use a thicker oil to slow it down."
Fact: Oil consumption is caused by worn piston rings, valve guides, or seals. A thicker oil may slightly slow the rate past worn parts, but it also increases crankcase pressure and can worsen leaks elsewhere. It treats a symptom, not the cause. - Myth: "A higher oil pressure reading always means better lubrication."
Fact: Oil pressure is a measure of resistance to flow. While adequate pressure is vital, excessively high pressure due to thick oil indicates poor flow and increased pump strain, not better lubrication. - Myth: "All oils with the same SAE grade are the same."
Fact: The SAE grade only defines viscosity. The performance specification (API SP, etc.), base oil quality, and additive package determine protection against wear, sludge, and oxidation. Two 5W-30 oils can perform very differently.
In conclusion, the pursuit of the "thickest car engine oil" is a fundamentally misguided approach for over 99% of drivers. The relentless advancement in engine technology and oil chemistry has moved towards lower viscosities for a reason: improved efficiency, superior cold-start protection, and adequate high-temperature performance through better base oils and additives. Your vehicle's manufacturer has already provided the optimal answer in the owner's manual. Deviating from those specifications, particularly towards an extremely thick oil like SAE 60 or 70, introduces significant risks with minimal to no benefits for a street-driven vehicle. Intelligent vehicle maintenance lies not in arbitrary choices, but in adhering to engineering specifications, understanding the principles behind them, and using high-quality oils that meet the precise demands of your engine.