Question: what is the difference between 10w 40 and 15w 40 oil?
Answer: Viscosity or thickness of the oil increases with rise in temperature. A 10W40 oil will be thicker in the cold than 15W40 oil, but they will have the same viscosities at higher temperatures. The winter viscosity number dictates cold-start performance. ... 10W40 oil will perform cold starts better than 15W40 oil.