Both Bowtieollie and Oil4kids are correct in my opinion. Lamars piston comment was very funny, big block Mopars, Caddys, and other engines have a HUGE 4.310 bore!. And don't require any special oils. As I've posted before, I put a stock forged 409 piston an a digital scale, then a stock forged 454 piston. within an ounce. If the engine is freshly rebuilt AND TO MODERN STANDARDS, synthetic should work fine. If it's used and moderate miles then a QUALITY 10-40 should work just fine. Anything else is just bloviating. There are tens of millions of engines on the road today that run just fine on good 'ol 10-30, and a W block is just another engine, albeit with a 74 degree head bank.
Here's a tip: ever see that magnetic sheet that is about a 1/16th of an inch thick and flexible, bendable?. Well cut yourself a piece and wrap it around your oil filter, then use duct tape to hold it on, collects iron particles. A magnetic drain plug helps to (after market or GM). And GM makes a truck oil canister (or oild filter) that holds around an extra 1/2 quart and the filter is 50% larger to boot.