I have similar era forks on my 70 T500 and I use 30wt oil which I understood to be the factory spec. I have read others use lighter oils or AT fluid. Damping is crude by modern standards, for street use not much benefit in using anything too fancy. MY 30wt oils came from a garden center sold as lawnmower engine oil.
Pick something (maybe not EP90 gear oil to start with ) see how it rides, adjust up or down to suit.
I use 20w50 engine oil as recommended by Crooks Suzuki many years ago, but if you want it a bit softer, use slightly thinner oil, always correct amount though.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
Thanks for the replies! Yeah, you're right there. Not many TC250's around for sure. I was more worried about damaging the old seals in place. I'll tear them down later and get new seals but just want to ride it a bit now.