Definitely wouldn't hurt to drain and refill the gear oil - you haven't changed the oil and the previous owner probably hasn't either............for years!!
Just another note.....
When you move your shifter across to reverse from the middle position between 1st & 2nd, does it feel like it moves thru a restrictive type notch? There is a spring type catch you have to "push" the shifter thru - it avoids you accidently catching reverse when you move the stick back from 1st to 2nd. If it's broken, there's potential to accidently move across & snag reverse......grrrrrrind
There's also a shifter centering spring that holds the shifter firmly inbetween 3rd & 4th when in the neutral position. ie: if you held the shifter in neutral between 1st & 2nd it should spring back to between 3rd & 4th. These tend to break over the years and can make shifting sloppy.
You can buy a shifter rebuild kit with all the bits in it