Cobb to answer your question.
The W indicates "Winter," indicating that the oil is multi-weight. The first number precedes the W and is the shear resistance, or viscosity, of the oil at 0 degrees F. The second number of the oil is the viscosity at 215 degrees F. That means that both oils are the same viscosity at start-up, but when they reach normal operating temperature, the 5w30 will be thicker. If your car specifies 5w20, you should be using what the manufacturer specify's for your vehicle.
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