You can buy from China though. And China is the largest import trading partner for the majority of countries in the world. They literally don't need to do anything to prop up a "petro-yuan".
So no one is going to use a controlled currency for a hard liquid commodity. So if China wants petro yuan they have to liberalize that, which will break their peg.
China could have more international trade in the yuan before all of Americas recent misadventures. But that has cast consequences for their economy, and possibly the ruling elites power structures.
- offshore yuan
- onshore yuan
The mainland vs offshore renminbi restrictions disappear in Hong Kong, Singapore, etc. where most mainland Chinese trading companies and otherwise have offices anyways. Trading offshore to onshore renminbi becomes their problem, one that they are fairly accustomed to.
This is as near an iron law as there is economics, you can’t keep a peg and have a large trade in a large liquid commodity market. China is trying to slowly thread this needle and they can get away with it with Iran and Russia because they are approaching vassal status because the petrodollars are closed to them. Everyone in the world can see this and wants to avoid it.
If you are an oil producer what you want is to diversify your currency risk. Right now China is _preventing_ this, because there is no way for them to become a major player in that market without huge impact on their economy and probably their political system.