MasterPull has some nice info available online. A M12000 can take 125' of 3/8" of regular synthetic line http://www.masterpull.com/cpage.cfm?cpid=502. I'd stick to 3/8" regular synthetic line @ 18,000 lb spec, or go with the SuperLine or SuperLineHD at 5/16" at 21,000 lb spec of you want either more room on your drum (helps with side pulls) or more line http://www.masterpull.com/cpage.cfm?cpid=197.