From Castrol's response it sounds like the change was made when Dexos1 Gen3 came out, it seems like they were coming out with GTX FS anyways so they opted not to recertify Magnatec for the newer GM spec. I've always wanted to try Castrol again but haven't since around the early-mid 2000's when I switched to M1 and then PP when it became all the rage on here, I've mostly been using Sopus ever since.