Had a 2010 Civic and best viscosity year round seemed to be 0w20. I used 0w20/0w30/0w40/5w30/5w40 synthetic oils in it and 0w20 was the best performer as far as throttle response and fuel economy go. With 0w40/5w40 the throttle response was noticeably slower, sluggish. Mobil 1 Advanced Fuel Economy 0w30 was a great performer as well though. But Idemitsu 0w20 was an absolute favorite in that engine: smooth, quiet, responsive, and efficient.