More than a dozen entities were sanctioned by the U.S. over their involvement in the shipment of Iranian crude oil and liquid petroleum gas to Syria recently.
The reason why Syria is buying Iranian oil is because the U.S. is currently stealing Syria's oil.

The Syrian Oil Ministry released a statement on 9 August accusing US forces occupying Syria of being responsible for the theft of most of the country’s oil.

“The amount of oil production during the first half of 2022 amounted to some 14.5 million barrels, with an average daily production of 80.3 thousand barrels, of which 14.2 thousand are delivered daily to refineries,” the oil ministry’s statement said.

The statement went on to say that “US occupation forces and their mercenaries,” referring to the US-backed Syrian Democratic Forces (SDF), “steal up to 66,000 barrels every single day from the fields occupied in the eastern region,” amounting to around 83 percent of Syria’s daily oil production.

According to the ministry’s data, the Syrian oil sector has incurred losses nearing “about 105 billion dollars since the beginning of the war until the middle of this year” as a result of the US oil theft campaign.

A top Pentagon official has acknowledged that Washington’s strategy is to starve Syria’s central government of revenue it needs to rebuild, after a decade of war fueled by foreign powers devastated the country.
Seemingly unaware of the irony, Voice of America even used the headline of US-Led Coalition Blocks Attacks at Conoco Oil Field in Syria.
Congress has voted on the issue of withdrawing the U.S. troops illegally occupying eastern Syria several times, but each time rejected overwhelmingly in bipartisan votes.

NPR stated clearly in 2020, “U.S. forces in northeastern Syria have a relatively new mission: securing oil fields not only from ISIS, but also from Syrian government and Russian forces”.

Recently arab tribal forces in the area have launched a low-intensity rebellion against their American occupiers.