As my user name suggests, used to be a huge fan of additives. Not as much these days, bc you can just change the oil for the price of a lot of them.
That said, I think some have real benfits - a moly additive like MolySlip or Liqui-moly has been shown to help quiet engines and reduce wear.
I think for engines that suffer from noise or low OP, STP blue-bottle is a long-proven additive that 'boosts' up the oil a bit w/o doing any harm.
Just put a 15oz bottle in with my oil change, and engine is a bit quieter, bit smoother, and starts a bit quicker from improved cylinder sealing.
For the small price, not a bad product.