Before you do anything, I'd try a different gear oil. Try to use one that says it's specifically designed for use in manual transmissions or transaxles. The ZF and RBT spec. calls for a GL-4 oil but many people use GL-5 oils without issue. Many 80W90 GL-5 gear oils are designed to be used in differentials only and are too slippery for use with synchros.
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