Hi ! Oils are very complex these days , but in a nutshell the 1st lower number is its effectiveness at low temp ie the 10 in a 10w/40 stands for -10,the w means winter! so that oil protects down to minus 10 degrees c, a 20w/50 protects down to minus 20 degrees ect. The higher number is the viscosity of the oil , all oil's are tested at 100 degrees centigrade so the lower the number the thinner the oil at this temp!ps a 10w/40 is thought to be the best oil for our climate.hope this helps but to be honest it's just the tip of the iceberg!!!.Regards Kev.