When you think of using oil additives you must start with an engine flush. My son is a good mechanic and rebuilds old cars to keep out of his wifes way.
He always flushes an engine when he receives  a car to fix, including a second flush with very light oil sae 10 or 20 with a NEW cheap filter run for about 5 miles very lightly or idle for 20 minutes if weather allows. He does not recommend any of the additives claimed to prevent wear or restore engines. They usually have PTFE "Teflon" in them which can settle into oil galleries as well as on bearing surfaces and cylinder walls. As a solid in suspension they also block your oil filter. Don't fall the 3 card trick and run heavier oils. They break down under heat and varnish and coke you engine. Accept the lighter 30 or 40 multigrade oil will burn and need topping up. Most of the "stop smoke" additives only thicken the oil and cause loss of oil pressure. He is a fan of synthetic oils and Mobil 1 is a good followup to mineral oil as it is not a true synthetic but derived from mineral oils and mixes with the dregs of the last mineral oil change. It gives smoother idling, runs the engine cooler (50% of heat is disserpated in the oil) and stays cleaner (no carbon production). It also lasts longer between oil changes, so the extra cost balances out.