Honestly run whatever the owner's manual calls for or what the oil cap says (if new enough). 10w-30 should be fine for most normal scenarios. If the engine burns excessive oil you might consider running thicker oil to slow oil consumption like 10-40 or 15-40, or even 20-50 in extreme cases, but that's usually as an engine is on its way out already. With a good running engine just run what's supposed to be in there, choose a high quality oil like Castrol if it makes you feel better. Running a thicker oil actually just makes your engine work harder/lose power, and doesn't lubricate as well because it can't get into as tight of spaces. The important thing is changing the oil and filter regularly every 2500-3000 miles (I know the user's manual said it's okay to change the oil every hundred million miles or whatever but go figure GM engines have a problem with rings as they get older would've thought... ::rolls eyes:: ). The whole shell Rotella thing is overblown IMO. It's a good oil and all, but it's non detergent oil being for diesels and all. If you have some super old fashioned engine made out of old metals and it's already in need of some love it is probably helpful, and it's also really good to run in motorcycles that have a wet clutch believe it or not (the detergents break down the friction material and make the clutch slip, almost all diesel oil is safe for bikes). But for a regular SBC there's no advantage to running a non-detergent oil, those detergents are in it for a reason they keep your engine cleaner. If you're not running some POS that's chugging oil and blowing smoke, just run regular oil
