At 180°F SAE 30 is 18.7 cST and 10W-30 is 17.5 cST so they're almost the same

Here's where you can graph them for yourself.
http://widman.biz/English/Calculators/Graph.html

I used numbers from Valvoline which you can find here:
http://www.valvoline.com/pdf/premium_conventional.pdf

1952 Chevy 3100
Eugene's Blog