I personally stick with the 5W30. I would not buy any high priced racing oil until you get all the junk out of that motor. The 5W30 oil was chosen for the LS1 because of the high tolerances within the engine. Using anything thicker will not lubricate as well.
Again use a cheep oil for cleaning the engine out, also get yourself some of those powerful surgical magnets and tape them to the outside of your oil filter. This will help get some of the junk out.
Drain your oil onto a large door magnet like a pizza delivery guy has. Every time you change your oil scrap the crud off and start over until you don't see anything else left in the oil. once everything is nice and clean put the good stuff in.
That's my best advice, I hope it helps