I change my oil after each DE. Call me anal, but I think once the oil gets really hot, it starts to break down(I'm using Kendall GT-1, 20-50w).
As far as oil cooling goes this is what I would use these oil lines:
http://www.elephantracing.com/oilhandling/oilines.htm
and this cooler:
http://www.elephantracing.com/oilhandling/elephantoilcooler.htm
Available at Pelican.
I would also recommend installing the numerical oil temp gauge. I though my SC was running way hotter than it really was. I don't know if the gauge was way out of wack or what. But once I changed to the numerical gauge, and took a reading with an I.R. gauge, I could see that my car was running way cooler than I originally thought.