I wonder if it's related to this article which said GM was in some instances seeing worse performance with Gen3 oils rather than Gr3+.
Group II+ and Group III+ base stocks do not officially exist as categories under the American Petroleum Institute’s (API) Annex E of API 1509, Engine Oil Licensing and Certification System (EOLCS), a voluntary licensing […]
www.fuelsandlubes.com
The Lubrizol Relative Performance Comparison chart shows better wear protection with Gen2 but Gen3 has better performance in terms of deposits and oxidative thickening.