If the noise is on startup from cold, I would not bother with any additive. The lifters or supports will leak down slowly when the engine is at rest. They should close up their clearance within a short time after the engine starts if oil pressure is adequate.
You can try an additive if there is 'ticking' at idle after the engine has run for a while. I would want to know the idle oil pressure before trying a mechanic in a can as adding anything except a viscosity improver to an engine with low pressure will likely not have a good effect. Most additives are a solvent of some sort, a highly volatile mix that will dissolve some deposits as the oil flows over them, kind of like a dishwasher but low pressure, and then will quit doing anything after it has evaporated and been fed to the PCV system and burned. The dissolved stuff will either stay in suspension until the next oil change, get caught in the filter(unlikely), or settle to the bottom of the oil pan or onto horizontal surfaces where the oil flows at low rates of travel. No place else to go...
If the engine has been maintained, it should not need the 'mechanic', as regular changes will help remove dissolved 'stuff', and prevent buildup in the first place.
If you hear the ticking at above-idle engine speeds, you might want to really take a look at oil pressure, using a gauge to know the 'real thing', or the flow to the cam area, or the lifter/supports them self. If they have leaky valves, they will allow good oil flow, but will not fully support the cam follower against the lobe(s), and you'll have tics.
tom
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