NOT motor oil! Jack oil is OK but you can get various grades from your local Motor Cycle shop, used for front forks, and this will be more suitable. Anything over 20w is likely to blow your seals. I'm sure you'll get more advice here.
I would contact someone who rebuilds them. I have a bottle that my father must have had for about 20 years for filling Luvax dampers. Its quite thick, so by the sounds of it you might need a thinner grade.