Question: what is the difference between 10-40 and 20-50 oil?
Answer: The smaller number, (the 20 part of 20w-50) describes how the oil behaves at 40 deg C and a 20w at 40 deg C is about 10 times thicker than your engine wants. A 10w-40 is better at around 6 times thicker and a 0w-40 is much better at about 4 times thicker.