Cecil-

If you use 20W-40 oil, it is 40 wt cold and 20 wt hot. The "W" after the first number refers to the viscosity at 120 degrees F. The other number is the viscosity at 32 degrees F. Oil cannot thicken as it warms up. So, that said, straight 30 wt will be thinner on start-up, and thicker at operating temp.

-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's