Why not just run Valvoline vr1? I know it's not synthetic, but if you're running regular Valvoline conventional with a zinc additive, vr1 already has the higher zinc you're looking for. Although I'm not an amsoil user, if you want a high zinc synthetic then amsoil z rod is available in 10/30. If you aren't stuck on a thirty grade, it's also available in 20/50 and you also have Mobil 1 15/50 as an option.