There are many stories about the difference differing oils can make in your fork. RockShox only recommends their oil in their forks. They also say you should only go with the oil weight recommended as well. For the most part, the plentitude of adjustments that are given should be enough to tune the fork to your needs. All of RockShox's current forks use 5 weight oil, but some older forks have different needs. And, just to prove there's an exception to every rule, 2002 Boxxers can have their oil weights changed. Before buying oil, consult your owner's manual, and, if you lost that, check the web. Once you have that info, order the correct weight. When adding oil, precision is usually a good thing; so a syringe or two-stroke oil mixer can help with the job.
Please click here for the RockShox oil level and weight table.