Boiled - it dries and so attracts less muck. Don't know if you're aware of precautions with used rags? The oil oxidises and can get rag hot if scrunched up so spread it out flat for a bit.
"Boiled" linseed oil is not actually boiled - it has driers added to it.
Boiling linseed oil is a dangerous activity - medieval varnish makers, who did boil linseed oil, were required to live and work outside towns because of the fire risk that their activities posed.