Whenever one discusses oil recommendations for the S2000 you really need to think about the oil technology when our engines were first developed and certified pre-1999. At that time a 5w40 oil would have meant that you were using a high quality full synthetic, and as such it would have been decent for cold temperature use. The average consumer likely used conventional oils when running a 10w30 viscosity, so 5w40 made sense as an optional Honda recommendation for cold weather use. Plus it also was a recommendation for other parts of the world.

Conventional 5w30 oils in 1999 were still questionable as most were being formulated using thinner base stocks to get the 5w visc. spec, and they would have higher amounts of viscosity index improvers to thick them up and achieve the hot 30 weight viscosity spec. VII's have poor lubrication properties, and when the oils broke down over time you could be left with a poor quality oil. Conventional 10w30 oils used thicker base stock oils and much less VII's. So Honda relied on a 10w30 and 5w40 recommendation to certify our motors. They kept the recommendations up to 2009 despite improvements in oil technology because it is too expensive to recertify a motor with other oils on a low volume car that was destined to end production.

Today's synthetic 0w-30 and 5w30 oils have some benefits over 10w30 without introducing any compromises. There is nothing wrong with 10w30, but 0w30 and 5w30 in a high quality synthetic oil can be just as good or better. I would use either of them without any concerns. When it comes to cold weather use, a synthetic oil will flow much quicker, and 0w and 5w synthetics will provide quicker oil circulation. Cold start engine wear is always higher than what you will see when driving down the road with a fully warmed engine and oil sump. If I drove my car in winter it would definitely have 0w-30 synthetic oil in it. I run 5w40 synthetic oil for 3 seasons, and the car is put away in winter.

 

now that's a mother fuggin answer to the question.