The point is that the Petrodollar system requires countries to buy US treasuries to be able to buy oil. That is what makes borrowing cheap for the US, and what keeps the dollar demand up.

I'm not clear on how the Petrodollar system actually works.

If oil is sold in dollars, that only has to affect dollar demand for the time it takes to transit through some other currency to dollars to oil. So however long that takes to settle.

Where do the additional demand components come from? Why do countries have to buy US treasuries to be able to buy oil? Don't they just have to use dollars for the transaction?