What makes you think the rebels are the right horse?
Besides, China does not need to arm the rebels to gain access to Saudi oil. Simply put, why would China break the only principle it adheres to in international relations over oil it can buy freely anyways?
Lets also not forget that China has sizeable investments in Syrian oil fields, which would be worthless if Assad falls or if Beijing decide to arm the rebels and get caught doing it. If oil and money was the only things China cared about, it should be arming Assad.
I think sometimes even my fellow Chinese members are sometimes brainwashed by the relentless western media propaganda onslaught into thinking China only cared about money and resources. China is actually one of the few powers today who actually keep their word, does wwhat it says and is consistent in the long term. Unless there is an official government announcement abandoning its none interference policy, China will never arm rebels in any country against their own government. The only exceptions might be in cases where the government is installed by a foreign power and is in effect a puppet regime to facilitate foreign occupation. But even then, China would need to have been mightily pevered by the foreign invasion to get involved so directly in the first place.