I was talking to a guy that rebuilds front and rear differentials for the past 30 years and he told me that synthetic oils are great. But not for the differentials. He said the reason why you shouldnt use it is because conventional gear oils push the contaminates to the side walls of the diff cases and it sticks there not allowing it to be continually ran through the gears and bearings. He said that he has seen bearings shot after about 50-60k miles because of this. He said that synthetic oils are the best for any system that is filtered but diffs are not. And the properties of the synthetic blends do not allow any wear contaminates to stick to the sides like the conventional oils. I guess if you continually change the oils maybe this statement may not apply. But then at what interval would be acceptable to not allow any of the contaminates to wear on the bearings ect. Just thought I would share an opinion.