In the beginning there were single weight oils. Single weight oils are thicker when cold and thinner as they heat up. So when the engine is cold you don’t get great lubrication and when it’s hot the oil thins out too much. So you chose a mid range oil as a compromise. Then multi-weight oils were invented. They act like a 10 weight when cold and a 40 weight when hot – the best of both worlds. Use the 10W-40W straight.
