Break in oil

Until just a couple years ago, most Rotellas were dual rated for gas and diesel. With the introduction of the API SP specs, Shell dropped it for some of them. They still have a good dose of anti-wear additives, and more calcium or magnesium detergents to help keep soot in suspension than gas only oil does. The calcium is bad for direct injection gas engines, causing pre-ignition. The ZDDP can be bad for the catalytic converter. So gas approvals have been removed. So if you don't have direct injection or cats, you're good to go.

Avoid using additives for this simple reason. You don't know what it will do. Will it help? Possibly, but it has a good chance of making other components in the oil less effective, making you worse off. Just get oil with what you need already blended in. Don't play chemist, there are lots of choices.

If you have a stock slant six or 2-barrel V8, just use a decent modern oil. Your valve spring pressure doesn't require anything more. If you have a performance engine try to use an oil with a little boosted ZDDP. The Mobil1 I mentioned earlier is a good one, as is their 15W-50. Valvoline VR-1 has some street grades with extra ZDDP. Remember that if the oil says "Resource Conserving" in the little donut, it cannot have the higher levels of ZDDP.

If you believe you need something with more, there are several out there, just maybe requiring to be shipped to you. Driven, Amsoil, Redline are a few. Summit has a large selection of lubes, but I'm not sure how much is actually in stock.