Where the myth comes from
The idea that thicker oil is better is easy to believe. Thicker oil clings to surfaces and feels more protective, and a heavier grade can quiet a noisy older engine for a while. But engine design has moved on. Modern engines use tight internal clearances and precise oil passages, and they are built and tested around one specific viscosity grade.
When you pour in oil that is thicker than designed, it does not flow as quickly at startup, a moment when fast oil delivery matters. It also takes more energy to pump, which can lower fuel economy. Thicker is not the same as better.
What the right grade actually does
The grade in your manual is chosen to balance two jobs: flowing fast enough when the engine is cold, and staying thick enough to protect parts when the engine is hot. Engineers pick a grade that does both for your specific engine and the climates it was designed for.
Using the specified grade helps oil reach moving parts quickly on a cold morning, keeps the oil pump working as intended, and supports the fuel-economy numbers the engine was tuned for. A heavier grade can upset that balance, even if it seems safer.
When a different grade is okay
Sometimes a manual lists more than one acceptable grade, often tied to temperature or driving conditions. Some manuals also note that a slightly heavier grade is acceptable for a high-mileage engine that burns oil. In those cases, choosing within the listed options is fine.
What is not a good idea is guessing on your own. If you think your engine needs a different grade, check the manual first, and treat its guidance as the final word. Remember to keep two ideas separate: viscosity is the grade, like 5W-30, while the specification, like an API or ILSAC standard, describes the oil’s performance level. Both should match what the manual calls for.