A few questions here, first how do you know when your lefty needs service/rebuilt, etc.? I have a 2005 Prophet 2000 with the Carbon Lefty Max 140 w/SPV and I have about 500 miles on it.
At 500 miles it is a judgement call. Cannondale says about every 40 riding hours, I generally say once a year is sufficient. That said, riding conditions are a huge part of the equation, the nastier, muddier or dustier your rides are, the more frequent you will want to think about it.
Second, does the lefty have oil inside? On a few occassions I've noticed what appears to be oil on the outside of the boot attracting dirt.
Yes it does, but it should not make a difference in plushness. Which boot, and where, top little breather hole boot, or lower ridged boot?
Third, my rebound adjustment still seems to make a difference in settings but it no longer clicks between settings - just very hard to turn with no click points.
This makes me think you ride in crappy conditions, as it is just ball bearings perched on top of springs inside little holes that make the clicks against the underside of the red cap.
Forth and last, can the (above) average guy rebuild the lefty? I've rebuilt Motorcross forks often and know my way around a toolbox quite well. Before I get in over my head I was just wondering if this is a project even worth undertaking or if special tools/equipment/parts are needed?
You will need some special tools, or time and creativity to get around that. Most shops will charge $25 to $35 to rebuild it, if they charge more, find out specifically why, as it is not that long a job. If you wish to do it, I can help you through it, but it may be easier to get it to a shop. I also offer the service at my shop, and many folks send them in, but I strongly encourage supporting you LBS, if they are competent with these forks, if not, contact me, and I'm glad to help. Good luck!