Which car you have as those numbers are showing the Temp related Viscosity of the Oils, as both the 15w-50 and 20W-50 are same but i think you checked 15w-40. Better for for 20W-50, and also they have 02 types one is normal and other is with Friction Controller Cera-tec type thing mixed which costs more. Please use Engine Flush prior to Oil change