If you change the oil you might be able to get some GL-4 manual transmission lubricant, which are supposed to work better than the GL5 multi-purpose stuff that most people (at least here in Taiwan) use, which has to be compatible with differentials..

Here's some deep-thinking (over-thinking?) on the topic.

https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/1231182/synchromesh-manual-transmission-lubricants

People speak highly of the Redline manual transmisssion stuff but I've never used it.Probably pricy

I got this "Multi-Purepose" BP stuff in Japan, probably not as good as a dedicated MTL, but the best I could do.

ecx.images-amazon.com/images/I/61jxvemQTDL._SY355_...g

If there is something mechanically wrong with your gearbox though (say a failing bearing) new lube won't fix it

Is the clutch clearing properly? Try bleeding the clutch through which will replace the fluid and get rid of any air in the cylinders and lines.