I tried to get this question out to the group but no one seemed to notice it ..... hence: How good or bad is it to mix GTX 5W-30 and Syntec 5W-50 ? I am aiming at increasing the viscosity for a piston slapping GM 3.1 engine. I also like the fact that GTX has some molly in it, does Syntec??. Are thetre a lot of Castrol mixers here? Please let me know what you think of it.