Depends what you want to do with it. For general use, a standard Stihl Rollomatic 20 inch Bar works just fine with a full comp chisel (RS). If you want a safety chain, use the RM style.
Personally, I use a Stihl 16 inch pro bar bar, ES type (removable tip, but more importantly, solid rather than laminated), a semi or full skip RS chisel chain. Blasts though the 8-12+ inch wood I use it for.
Out here we use only 3/8 chain. If you're out East (East of the Columbia river, which isn't very far East!), .325 is likely more popular.
You can find the equivalent in Oregon bar/chain, but unless you live where there is no Stihl dealer, I'm not sure it makes any difference at all (but I sure some on AS will differ!).
