All of Shell gas is Top Tier however the premium have the highest level of additive. I was searching around for this information too and this is what I have found.
„ Among samples of regular unleaded
gasoline (those with an octane
rating of 87), Exxon had the highest
level of additives (20.0 milligrams per
100 milliliters), closely followed by BP
(17.2 mg) and Shell (16.2 mg). Trailing
substantially were Citgo (6.0 mg) and
Pilot (5.8 mg).
„ Among premium fuels (92
or 93 octane), Shell took the top spot
(31.0 mg), followed by BP (26.4 mg)
and Exxon (21.2 mg). Citgo (9.4 mg) and Pilot 92 (8.8
mg) lagged behind the other three brands — including
the trio’s regular gasoline samples.