Mixing different engine oil grades
Multigrade oils have their viscosity measured at 20degC and 60degC (IIRC! – it’s been a while since I done it), and are matched to the equivalent SAE grade of oil at that temperature. For example, a 10W-40 oil, at 20degC is equivalent to a SAE 10 oil, but when heated to 60degC is equivalent to a SAE 40 oil. As Chris says, “Yes it is possible to alter engine oil viscosity by mixing two different viscosity grade engine oils. However, you cannot accurately predict what the result will be just by the numbers on each bottle and the mixture ratio.”