Welcome to Just Commodores, a site specifically designed for all people who share the same passion as yourself.
An oil change is a good idea. My gears freed up after I put penrite in so it will be worth it regardless imo.
Could be clutch related sounds like it might not be fully engaging so check your fluid first of all. Worst case is you're slave cylinder or pressure plate are gone. One thing though I find is when I haven't fully engaged my clutch is my gears grind.
And I guess worst worst case scenario is your syncros have had it but hopefully not.