Best point in Dexos is reduction in formation of deposits from increased solvency (detergency without much more of detergent add), from the syn part base, and better compatibility with oxigenated fuels (E10 to E85) by wider emulsification capability. Even though a GTL makes less deposits than any dexos ... It's all very mild improvment in relation to WSS-M2C946 motorcraft specs, only considerable at longer drain intervals. In shorter drain, both gives clean engines. That's from an oxidation perspective. In spite the biggest cause of sludge in modern oils are mostly external from the lube system (soot contaminants + solids + heat = sludge).