What the grades mean
10W-40 and 20W-50 differ in both numbers, so they behave differently cold and hot. The first number with the W describes cold-start flow: 10W flows more freely at low temperatures than 20W, so a 10W-40 engine circulates oil faster on a cold morning. The second number describes thickness at operating temperature: a 50 is noticeably thicker when hot than a 40.
That makes 20W-50 the heavier oil on both counts — slower flowing cold and thicker hot. 10W-40 is the lighter, more general-purpose grade. Thicker is not automatically better; a heavier oil only helps where the engine and operating conditions call for it.
Which one your engine needs
Use the grade printed in your owner’s manual. 10W-40 suits a wide range of general-purpose engines and tolerates cold starts better. 20W-50 shows up most often in classic engines, in some high-performance applications, and in very hot climates, where a thicker hot film can match the engine’s design or the heat it operates in. Some older engines have wider internal clearances and were built around heavier oil.
These grades are not freely interchangeable. Running 20W-50 in an engine that asks for 10W-40 gives a thicker film than intended and flows poorly on cold starts, which can slow oil delivery at startup in cool weather. Running 10W-40 where 20W-50 is specified may give a thinner hot film than the engine was designed for. Climate matters too: 20W-50 is a poor choice for cold regions because of its weaker cold flow.
When the manual lists more than one acceptable grade or sets it by temperature, follow that guidance. Otherwise, match the grade on the page — it already reflects your engine’s design and the conditions it was built to handle.