What actually changes
The two numbers describe different conditions. The first number with the W describes cold flow, and the second describes thickness at full operating temperature. With 10W-40 and 10W-30, the first number is identical, so cold-start flow is unchanged. The difference is entirely at the hot end: 10W-40 is thicker than 10W-30 once the engine reaches operating temperature.
A thicker hot grade is not automatically an upgrade. The 30 grade your manual specifies was matched to your engine’s bearing clearances, oil pump, and oiling passages. A thicker 40 grade does provide more film at temperature, but it also brings more internal drag and can flow more slowly into tight clearances and to components that depend on prompt oil delivery. That trade-off is why moving up a hot grade should follow the manual rather than a general belief that thicker means safer.
Before you switch
Let the manual decide. Some manufacturers list both 10W-30 and 10W-40 and tie the choice to conditions such as high ambient temperatures or higher-mileage engines. If your manual names 10W-40 as an allowed grade for your situation, using it is reasonable. If only 10W-30 is listed, stay with it, because the thicker grade can change oiling behavior in ways the engine was not designed around.
Warranty and emissions matter here as well. Manufacturers often tie coverage to specific approved grades, so only move to 10W-40 if it appears in your manual. Check the required oil specification or approval code too, not just the viscosity, so the oil meets the performance standard the engine needs. Matching the listed grade and specification keeps you within the maker’s guidance.