Although I'm not optimistic about Saudi trading oil for yuans. Won't hold my breath till when that happens. Very unlikely.
1. It only makes sense. China gives RMB to Saudi Arabia for oil. Then Saudi Arabia gives RMB to China for consumer goods. As of today, China is already Saudi Arabia biggest trading partner.
2. The Kingdom wants to diversify away from petroleum industry, and they have set up sovereign wealth funds to do that. If the Saudis want their fund to make investments into America, they will need USD. If the Saudis want their fund to make investments into China, they will need RMB. These two financial markets are the number 1 and number 2 in the world in capitalization.
For Saudi Arabia it is just a practical matter. Why fiddle around with the exchange rates of USD-RMB-riyal, when it comes to China, all Saudi Arabia really needs is RMB and riyal.