Question: is 20/50 oil thicker than 10/40?
Answer: the huge problem is the other number though. 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.