Rebound? bouncy rear end

I recently got my bike going again. I noticed the rebound damping is kinna non existant for the first few inches of travel. I see some seepage from the rear shock and suspect low oil level is the culpret. Can I add some oil myself, or is the shock charged with gas? I've never serviced a rear shock. I'm a mechanic by trade so no repair scares me, but I don't have any equipment to charge gas nor the gas to charge with. anyone got a suggestion.............thanks and happy trails. :think:

I went riding this weekend, I just added some power steering fluid to the shock, made a world of difference, I think I'm gonna rebuild it myself and find a place to charge the gas for me.