Is there an engine oil additive that actually will stop oil leaks - head gasket/valve cover gaskets/cam gaskets etc. without damaging the engine in any way?
Be sure to use the right specification oil. In the 80’s, there were cars that recommended 30WT in locations where the temperature doesn’t go below 30 degrees F. My Mazda truck recommended 30 or 40 in the summer. None of that 5W or 10W stuff. Thicker oil may slow the leaking. Additives cost too much if they are just thickeners.
Only option I’d try is some of the oil for engines with lots of miles, in the highest weight within the manufacturers specs.
If you want to try using the high mileage oil that is designed to help some problems, it may work, for a while. It does not really fix it however, but rather it patches it for a while, if you are lucky.
If those kind of fixes really worked, they would put them in all the fluids all the time.