Thoughts about using a 15w-50 synthetic oil in a 2009 6 speed transmission? Kinda like a jungle with scattered info. Some say GL-5 rated oils (75w-90/140)are not neccessary and maybe too much. Amsoil is the only company I see that rates their 20w-50 m/c oil as GL-1. Mobile says the 20w-50 m/c M1 syn is better than their 75w-90 gear lube, as does Amsoil. Lastly, the viscosity chart would show a 15w-50 oil actually closer in spec to a 75w-90 gear oil than a 20w-50 oil. Reason is I can get a qt of M1 15w-50 for $5 from a buddy. Heck, I'm sure it would be just fine but just throwing out for opinions...
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