Can different grades of engine oil be mixed?

[MacBethany]

Different grades of engine oil should not be mixed, as mixing may alter the viscosity of the oil and adversely affect engine lubrication. Engine oils are specifically classified into 4 types for summer use, 6 types for winter use, and 16 types for all-season use. Summer oils are graded as 20, 30, 40, and 50, with higher numbers indicating greater viscosity and suitability for higher maximum temperatures. Winter oils are graded as 0W, 5W, 10W, 15W, 20W, and 25W, where 'W' stands for Winter. The smaller the number before 'W', the lower the low-temperature viscosity, the better the low-temperature fluidity, and the lower the minimum applicable temperature.