This is what I use:

http://www.motorcycle-superstore.com/1/4/7...d-Gear-Oil.aspx

It is available in a GL-5 90w, exactly what Honda recommends.

I buy it at the local motorcycle dealership. It is always in stock.

 

That looks like it will work well in the S2000. Let us know what you find on the drain plug and what condition the oil is in when you drain it. If you have an extra $22.50, send a sample off to Blackstone Labs so we can see how it held up. Thanks.