Providing at least some short-term relief, the combined deliveries at the end of June aboard the two Iranian tankers amounted to around 100,000 tonnes, according to records provided by a shipper and satellite tracking data, Reuters reported.
A third vessel appeared to be on its way to Syria along the same route, but then dropped anchor off the coast of Port Said in Egypt, shiptracking data showed.
At current market prices, the two shipments are worth around $90 million in total.
Syria's need for gasoil makes it a perfect fit to swap with Iran, which produces plenty of diesel but has struggled for years to find sufficient supplies of gasoline, the fuel used in most civilian cars.
The second vessel was still berthed at the Syrian port of Banias on Tuesday, and expected to load a combined cargo of heavy and light crude oil from two Syrian oil terminals.
It was not clear who would purchase the cargo. Both Iranian and Syrian crude oil is subject to Western sanctions, and Iran itself is already facing a dramatic and potentially crippling cut in its own exports due to the embargo, which came into force on Sunday.
The last exchange of petroleum products between Syria and Iran took place at the end of March and into April, when the same vessels made the journey from Iran to Syria and back.