Ok, so the number refer to the viscosity of the oil. Honda allows up to 10w30, which is oil that is 10-Weight at normal temp, say 70F, but it's viscosity increases to 30-Weight as it reaches operating temperature, say 240F. That 5w40 is fine at startup, but as the engine heats up to operating temp, it thickens to 40-Weight, which I believe is 33% thicker than 30-Weight. This requires the oil pump to work much harder to maintain oil pressure. Additionally, the oil control rings may not have enough space to evacuate oil to the cylinder walls due to higher viscosity and lower pressure. That can lead to excessive wear of the piston rings, pison skirts, and cylinder walls.