5W-30 describes how the oil flows. The 5W is how easily it flows on a cold winter start. The 30 is how thick it stays once the engine is hot. Two bottles can both say 5W-30 and still be the wrong oil, if the approval on the back doesn't match your car.
The approval is the real spec
Look for the car maker's code next: BMW LL-04, VW 504 00, MB 229.51, or an ACEA class like C3. That code says the oil is allowed in your engine. The grade only gets you to the right shelf.
Check two codes on the bottle
Both must match what your car needs. One without the other is the wrong oil.
Why low-ash matters
Low-ash oils (often ACEA C2 or C3) protect the particulate filter. Pouring a high-ash A3/B4 oil into a car that asks for C3 can clog that filter over time. If your owner's manual and the filler cap disagree, follow the manual for your engine.