What is the correct viscosity engine oil to use an an '07 S-2000 only driven in the summer?

 

The 2000 Service Manual calls for 5W-40 across the full range of temperatures, and 10W-30 for warmer temperatures (down to -20°C, which is just below 0°F)

I've used Amsoil 5W-30 for a while, especially when my '00 S-2000 was only driven in the summer. The F20C seems to burn this oil faster than my other cars (but I was using 20W-50 in my CRX Si for a while). I haven't tried other weights recently, so I might switch to 5W-40 or 10W-30 and see if there is any difference. I'm sticking with Amsoil, though.

 

10w30 in a good synthetic will fit your needs fine for street driving conditions.