Putting 5W-30 motor oil in an engine designed for 5W-20 is generally not recommended for long-term use. While it's unlikely to cause immediate catastrophic damage if you do it once, consistently using the thicker oil can lead to reduced fuel economy, increased engine strain, and potential issues with lubrication flow in tight engine clearances. The best practice is to always use the oil viscosity grade specified in your vehicle's owner's manual.
The key difference lies in the oil's viscosity, or thickness, at high operating temperatures (the '30' vs. the '20'). A 5W-30 oil is thicker when your engine is hot. Modern engines engineered for 5W-20 have very precise oil passages and components designed for the flow characteristics of that thinner oil. Using a thicker oil can cause the engine's oil pump to work harder to circulate it, leading to a slight but noticeable drop in miles per gallon. More critically, on a cold start, the oil may flow more slowly to critical upper engine parts like the valvetrain, increasing wear over time.
There are rare exceptions. For an older, high-mileage engine that has developed internal wear and is burning or leaking 5W-20, a mechanic might suggest moving to a slightly thicker oil like 5W-30 to help maintain oil pressure and reduce consumption. However, this is a diagnostic band-aid, not a performance upgrade. For the vast majority of vehicles, adhering to the manufacturer's specification is the safest bet for engine longevity and efficiency.
| Oil Grade | Viscosity at 100°C (cSt) | Primary Use Case |
|---|
| 5W-20 | ~8.6 - 9.3 | Modern gasoline engines, optimized for fuel economy |
| 5W-30 | ~9.3 - 12.5 | Broader application, some high-performance or older engines |
| 0W-20 | ~7.5 - 8.5 | Very fuel-efficient, cold-climate engines |
| 10W-30 | ~9.3 - 12.5 | Older engine designs, less common today |
