5W-30 or 5W-40: which does my car need?
The two most commonly confused grades on UK shelves, and why the answer is never 'whichever is on offer'.
The difference in practice
Both flow the same at cold start. At operating temperature the 40-weight is thicker, holding a slightly stronger film under high load and running slightly higher oil pressure. That is an advantage in a hard-worked older engine and a disadvantage in a modern one designed around a thinner film.
Why modern engines specify 30
Tighter bearing clearances, variable valve timing and fuel-economy targets all favour a thinner oil at temperature. Many engines from 2005 onward also route oil through solenoid-controlled passages that assume a specific flow rate.
When 40 is the right answer
Older performance engines, some diesels, and engines specified for it from the factory. Track use and towing also push oil temperatures high enough that the extra film strength is worth having, provided the manufacturer permits the grade.
The honest rule
Use what the manufacturer specified for your engine code, not for your model name. Two cars with the same badge and year can specify different grades depending on which engine is fitted.
A registration lookup returns the grade, approval code, capacity and interval for your exact engine — free, and without a sign-up.
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