My manual says to use 5W-20 year round. 5W-30 is ok in a pinch, but not recommended in lieu of 5W-20. (I have the V4 VTEC).

The reason: 5W-20 is basically hydraulic oil used in braking systems, fork trucks, etc. It has a lower viscosity than 5W-30, which has a lower viscosity than 10W-30. Thus, it is more fuel efficient - less resistance to flow. However, until recent Honda R&D, 5W-20 would break down (thermally) inside the engine and higher weight oils were necessary.