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5W-30 is the preferred oil viscosity
The 5 and 30 refer to the relative viscosity of the oil when it is cold (5W) and when it is hot (30W). A 5W-30 oil behaves like a 5W thin viscosity oil when the engine is cold, and it behaves like a 30W thicker viscosity oil when the engine is hot. A very thin viscosity is undesirable in a hot engine. In very high temperature situations, an even higher viscosity range, up to 50W, might be called for. Always go by the engine manufacturers recommendations.
The wrong viscosity of oil in your engine.
5W30
30w
The viscosity of the oil may not be heavy enough to protect the engine.
As the engine wears, the bearing clearances become a little larger, REQUIRING that you use a higher viscosity oil to maintain oil pressure. If using the recommended oil viscosity results in low oil pressure, you should certainly switch to the 10W40. HOWEVER, if the engine can maintain oil pressure using the recommended oil viscosity, you will get better economy and the engine will perform better.
Of course you can but when you do you are changing the viscosity of the oil. Your engine requires a certain weight or viscosity of oil. Use only the weight oil recommended by the manufacture.
viscosity
viscosity
5w-30
Viscosity breakdown