While the underlying engine might be very similar, Honda likely made refinements between model years that allowed for the use of a lower-viscosity oil (5W20). These refinements could include:
* Improved oil pump design: A more efficient oil pump could adequately lubricate the engine even with a thinner oil.
* Tighter engine tolerances: Manufacturing improvements resulting in closer tolerances between moving parts reduce friction and allow for the use of a lower viscosity oil.
* Changes in materials: Use of newer materials with lower friction coefficients could contribute to the ability to use a thinner oil.
* Emission standards: The switch to 5W20 could be partially driven by stricter emission regulations, as lower-viscosity oils can improve fuel efficiency, leading to lower emissions.
In short, although the engines might appear identical on paper, subtle engineering changes allowed Honda to confidently recommend the thinner 5W20 oil in the 2001 model year for improved fuel economy without compromising engine protection. Always follow the manufacturer's recommendations in your owner's manual, as using the wrong viscosity oil could lead to engine damage.
