Do we know how Costco's "additive package" rates relative to other providers (e.g., Shell's) additive packages?

Currently I buy Shell gas b/c I trust that it's good for my car's engine. Can I trust that Costco's gas is as good as Shell's?

You should be looking for "top tier" gas, which Costco sells.

	▲	edot 38 minutes ago \| parent [-]
		Correct. Beyond that, doesn't matter. Go to where the fuel is the freshest (hasn't collected water, sat in a rusty tank for a long time, etc.). So, also probably Costco.