Help with oil question
 

My engine has on it 420 miles. Engine was built by speed inc. The oil in the car according to the receipts is RP 10w40. I'm about to change the oil at 500 miles as instructed by Tom at Speed inc. Now, I have read a lot of bad things about RP and good things. I have read about weight and reports from labs. I live in Illinois and the temp I drive the car in ranges from 50f-100f. Car is stored in winter. What (as of today) is the preferred oil for a cammed built LS1 with my climate and driving temp? No track racing but I do get on it every once and a while. I have read 40 is no good in an LS1. I read RP sucks now as does Mobile 1. I read 5w30 is good for this motor all the way to 20w50. I have read viscosity reports and can't seem to get a clear picture of what to buy. Is Royal Purple a bad oil to use? 5w30 too thin for built motor? What brand? Help!!!

I don't want to doubt Speed Inc on their oil choice but the guy I bought the car off of said he would never run RP again because it is too thin. I'm lost as to what to do......