Engine oil is one of the most discussed maintenance products in the automotive world, which also means it has accumulated plenty of myths over the years.
Some of those beliefs were based on older engine designs, older lubricant technology or service practices that made more sense decades ago. Others were never especially accurate to begin with.
Engine-oil colour, brand type and old rules of thumb do not tell you everything you need to know. The correct oil and service interval depend on the engine, operating conditions and lubricant specification.
Myth 1: Engine Oil Must Be Changed as Soon as It Turns Black
Fact: Dark engine oil does not automatically mean the oil is worn out.
Engine oil naturally darkens in service as it is exposed to heat and combustion by-products. Detergent and dispersant additives also help keep contaminants suspended in the lubricant instead of allowing them to accumulate on engine surfaces.
As a result, oil can become very dark while still remaining suitable for continued service.
The opposite is also true: oil that still looks relatively clean is not necessarily in perfect condition. Oxidation, fuel dilution and additive depletion cannot be accurately judged simply by looking at the dipstick.
For more information, see Why Does Engine Oil Deteriorate?
Myth 2: Engine Oil Must Be Changed Every 5,000 km
Fact: There is no universal 5,000 km oil-change interval.
Modern engine designs, lubricant technology and oil-life monitoring systems have made fixed kilometre rules far less useful than they once were.
Some vehicles may require relatively short intervals under severe operating conditions, while others are designed for considerably longer service periods.
The correct interval depends on the vehicle, lubricant and operating conditions rather than one number that applies to every engine.
Start with the vehicle manufacturer's service requirements, then follow any specific service recommendation provided for the lubricant you are using.
See our guide to how often synthetic engine oil should be changed for a more detailed explanation.
Myth 3: You Can't Mix Synthetic and Conventional Engine Oil
Fact: Synthetic and conventional engine oils are generally compatible and can be mixed.
Both types of engine oil consist of base oils combined with additive systems. Synthetic-blend engine oils are themselves formulated using a mixture of synthetic and conventional base oils.
That does not mean mixing two oils is always the ideal approach. Combining products can dilute the performance characteristics of the higher-performing lubricant, and two oils with different specifications may not provide the same result as a product specifically formulated and tested to meet the engine's requirements.
For routine servicing, it is better to use one suitable engine oil of the correct viscosity and specification rather than intentionally mixing products.
Myth 4: Once You Switch to Synthetic Oil, You Can't Go Back
Fact: Switching from conventional oil to synthetic oil does not permanently change the engine or prevent the later use of a suitable conventional lubricant.
The important issue is not whether the previous oil was synthetic or conventional. It is whether the replacement oil has the correct viscosity and meets the performance specification required by the engine.
There is therefore no technical rule requiring an engine that has used synthetic oil to remain on synthetic oil forever.
In many modern applications, however, synthetic oil may provide useful advantages in areas such as oxidation resistance, cold-temperature performance and high-temperature protection, which may be why an owner chooses to continue using it.
Myth 5: Synthetic Oil Causes Engine Leaks
Fact: A suitable synthetic engine oil does not normally create a leak in a mechanically sound engine.
This myth is partly associated with older or neglected engines where accumulated deposits may have formed around hardened, shrunken or damaged seals.
A high-detergency lubricant can gradually remove some deposits, which may reveal an existing sealing problem that had previously been obscured.
In that situation, the lubricant has not created the damaged seal. It has exposed a mechanical condition that was already present.
Modern engine oils are formulated for compatibility with suitable engine-seal materials, and dedicated high-mileage oils may include enhanced seal-conditioning additives for ageing engines.
See Is High-Mileage Engine Oil a Gimmick? for more information about ageing seals and older engines.
Why Do Engine-Oil Myths Persist?
Engine and lubricant technology change faster than many maintenance habits.
Advice that may once have been reasonable for an older carburetted engine running an older lubricant can continue circulating long after vehicle technology has changed.
The same problem occurs when a recommendation that applies to one specialised application is treated as a universal rule. Racing oil, high-zinc oil, high-mileage oil and extended-drain synthetic oil can all be excellent products when used in the applications for which they are designed.
The safest approach is to select oil according to the requirements of the engine rather than relying on general rules or assumptions.
Engine oil should be selected and serviced according to the vehicle, lubricant specification and operating conditions. Colour alone does not determine oil condition, synthetic oil can be mixed with conventional oil, switching back is possible, and synthetic oil does not normally cause leaks in a healthy engine.
To find the correct engine oil for your vehicle, use the AMSOIL Vehicle Finder. For more information about API, ILSAC, ACEA and manufacturer requirements, visit our Engine Oil Standards Guide.
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