ISIS is funded by 1.2B Muslims in the same sect. The oil is a drop in a bucket.
It is true that some wealthy individuals from the Gulf have funded extremist groups in Syria, many taking bags of cash to Turkey and simply handing over millions of dollars at a time.
This was an extremely common practice in 2012 and 201
http://www.bbc.com/news/world-middle-east-29004253
“There has also been a long-standing flow of donations from sympathetic private individuals in the Gulf, especially in Saudi Arabia, Qatar and Kuwait, to the organization, which the U.N. Security Council Resolution of Aug. 15 (adding six individuals to the al-Qaida sanctions list) tried to address.”
http://www.pbs.org/newshour/updates/how-is-islamic-state-funded/