What the grades mean
15W-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. 10W flows more freely at low temperatures than 15W, so a 10W-40 engine 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, climate, and the engine type. 15W-40 is a long-standing heavy-duty and diesel grade, common in commercial engines and in hot climates where its cold-flow limitation rarely matters. 10W-40 appears in many petrol and general-purpose engines and tolerates cold starts better.
Specification matters as much as viscosity here. Diesel and heavy-duty engines often require oils meeting particular industry or maker specifications, and 15W-40 products are frequently formulated to meet them. Matching the grade alone is not enough if the manual also names a specification — check both.
If your manual lists one grade, follow it. If it allows either or sets the choice by temperature, lean toward 10W-40 where cold starts are common and toward 15W-40 in consistently warm conditions or where a diesel or heavy-duty spec calls for it. The grade and spec on that page reflect your engine’s design and intended use, so match them rather than guessing.