For me and my bikes, the thinner gear oils (70W90, etc.) did not seem to provide the "cushion" this old girl seemed to need for smooth gear transitions, especially towards the end of a ride.
The other part of the equation is learning to shift that clockworks gearbox:
1) (from a 1935 article) Clutch in
2) Toe into next gear & hold with foot
3) clutch out, see if it really "made the trip"
4) release toe pressure, crack on!
Don't get in a hurry - there's biggo pieces moving slow in there, it's not a Honda/Yamaha/Kawasaki/Suzuki...