answer 1
Short answer; no it wont harm the engine.
The numbers in engine oil are related to the viscosity, or thickness, of the oil at certain temperature. The difference between 5w40 and 10w40 is that the 5w will flow slightly better at lower temperatures, which is a helpful with the winter we've just had. So 5w is fine when youre recommended 10w40.
But for future reference it wouldn't work the other way around. Using 10w when 5W is recommended could harm your engine as the oil wouldnt flow as it was needed too at lower engine temperatures, ie when the engine is first started, which is when wear is at its greatest.
Hope that helps
Top 1000 Contributor
answer 2
Apart from the fact that one is a fully synthetic product and the other part-synthetic the main difference between the two products is how well they perform at low ambient temperatures or when the bike is started from cold.
A 5W oil will thicken up less than a 10W product as it gets cold. This means that a 5W will move around the engine easier than a 10W in cold (start-up) conditions which in turn means that there is likely to be less wear in the critical first few minutes after the engine is turned on.
Fortunately a 5W40 and a 10W40 will both have approximately the same thickness when running at the normal hot operating temperature.
In practice this means that a 5W40 is generally a very good substitute for a 10W40 (of the same specification) though not the other way round.
answered 1 year, 9 months ago