I use 20 wt oil in my SUs. It is available at most motorcycle shops as fork oil. Shouldn't cost as much as the SU or Stromberg oil at Vic British or Moss Motors.

Anything over 20 wt gives too much damping and results in a richer mixture on acceleration. Shouldn't affect it under cruising conditions though. Make sure everything is clean and free to move.

Just my $.02,
Wes