What the grades mean
5W-40 and 10W-40 share the same second number, so they reach the same thickness once the engine is at operating temperature. A 40-grade oil behaves the same when hot whichever of these you choose, which means hot-running protection is effectively matched between them.
The difference is the first number with the W, which describes cold-start flow. 5W flows more freely at low temperatures than 10W, so a 5W-40 engine cranks and circulates oil a little faster on a cold morning. Neither oil is thicker when hot; the only practical gap is how easily each flows before the engine warms up.
Which one your engine needs
Use the grade printed in your owner’s manual. Because both are 40-grade hot, the choice comes down to cold-start behavior and climate. 5W-40 is the stronger choice in colder regions, where its easier cold flow helps oil reach moving parts sooner after a cold start. 10W-40 is well suited to milder winters and appears in some older engines.
Many manuals list one of these grades specifically, and some allow either or set the choice by temperature range. If yours permits both, 5W-40 gives a margin in the cold without changing how the oil protects when hot. If it lists only one, follow that.
Switching between them is often straightforward when the manual allows it, since they match when hot and differ only cold. Still, confirm against the manual rather than assuming. The grade on that page reflects your engine’s design and expected climate, so match it — and lean toward the lower cold rating if you regularly start in cold weather and the manual permits it.