I have never really understood how dollar denomination of petro contracts would really make that much of a difference.

it seems folks could simply acquire USD shortly before spending the USD on oil. And conversely Saudi Arabia can sell there USD the day after making the purchase.

Is it just that folks tend to hold onto what ever currency they used, thus it is that tendency that causes all the extra USD demand from the petro dollar?

And if China were to have a gold conversion for the RMB won't this just drive the price of gold up not the Yuan since few will trust holding the Yuan for long periods?

But really why hold USD for long periods, since over time golds price is known to go up relative to dollars, why not just hold that?