Synchronizers don't fix themselves. If the clutch is working properly once the trans grinds going from one gear to another it will continue to do so.
It doesn't go away. If the grinding is gone the clutch wasn't depressed enough when the shift took place. If it grinds in every gear it is likely the problem is the clutch.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas