Here is a good site with oil info.

http://theoildrop.server101.com/cgi/ultimatebb.cgi

I'm no oil guy but where I live temps can drop to -40.

On a small block chev with a quality mech. oil pressure guage I can see how long it takes for 5w30 dino to make pressure vs. 5w30 synth. and at temps below about -10 F the difference becomes blatantly obvious. If I were cheap I'd run dino in the summer but I can't be bothered.