It's probably better to already have the additive in the oil if possible. I usually use SL and older spec oils but only have so much old stuff in my stash. When it's used up that's it. At $1 a quart or less in many cases (most purchased on closeout), that's my best bang for the buck in oil.

For an additive to newer spec oils I like to use SLOB.
CD2 Street Legal Oil Boost. I bought a bunch of it on closeout for 50 cents a pint. Unfortunately it is not made anymore but while it was around, it was the best bang for the buck in an additive.

Here's an analysis of one pint to give an idea.
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Parameter: VOA Data (Manufacturer's data)

Zinc: 4898 ppm
Phosphorus: 4921 ppm
Calcium: 4279 ppm
Moly: 0 ppm
Vis @100C: 29.5 cSt