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Is it ok to switch weights of motor oil, for example go from a 5w20 to a 10w30?

It depends. Some vehicle manufacturers provide a range of recommended motor oil viscosity grades based on the outside temperature in which the car is driven. Other manufacturers recommend the use of only one viscosity grade. For best engine performance, follow the manufacturer's recommendations.

Is it ok to use 5w30 in a car if the owner's manual calls for 5w20?

Some manufacturers now use 5w20 engine oil to maximize fuel economy for certain applications. Using a 5w30 may cause a slight decrease in fuel economy. For maximum engine performance, Valvoline suggests following the recommended motor oil viscosity and maintenance schedule provided in your owner's manual.

What does the "W" in a grade of motor oil stand for?

The "W" in motor oil stands for winter.

Is thicker oil better?

Depending on the circumstances, thicker oil can be used to help fill in wear gaps that have developed over the years. However, the recommendation is to always use the viscosity recommended in your owner's manual.

When does it make sense to use straight weight oil (SAE30) versus a multigrade oil (5w30)?

A straight weight oil is never recommended for use in a system that requires a multi-viscosity oil. Straight weight oils are generally recommended for smaller engine applications or older vehicle applications that were made before multi-viscosity oils were produced.

Can switching from a conventional to a high mileage motor oil create gasket leaks?

No. The conventional and high mileage oils are compatible with each other. Mixing or switching the products will not cause leaks.

Is switching types of motor oil (conventional, synthetic, etc.) harmful to my engine?

Conventional, synthetic blend, synthetic and high-mileage motor oils are compatible and will not harm your engine.