Trans oil and additive for a 67 GMC P/U with manual trans?

I just got my 67 GMC pickup on the road after 9 years. It's got the stock 4 speed (with granny gear) manual trans. I changed the trans oil to a redline product- I think it was MT 90- back in 2002, and the truck has been parked in Central California since then. I don't think the MT 90 made a difference in the 500 miles or so I drove it after changing to MT 90 in 2002.

Should I change the trans oil again? Is there an additive that might help with the grinding? I'm guessing it's synchros going out. I'll probably be changing the trans to something more modern in a year or two, so dont plan on any repair work to this trans.

Thanks!

I like the Royal Purple SynchroMax fluid personally. That's what I use in my Jeep six-speed and it smoothed it out some.

I use AMSOIL for all my vehicles