He believes the Gulf states would have adopted the practice earlier in the war if not for the intervention of the White House.
By issuing assurances that the conflict would be a brief one and Iranian defiance short-lived – crushed, variously, by a decisive military victory, negotiated peace or economic warfare – it stopped the oil-rich countries improvising.
“The White House keeps telling us that this will be over next week,” Mr Johnston’s contacts kept telling him, when he asked why they were not taking the gamble to get across the strait.
Eventually they lost patience.
“It was the White House’s own routine and kind of repeated declarations that this was short-lived that actually prevented this risk tolerance behaviour from emerging,” he said. “Now I think that no one believes the White House any more.”
According to data firm Kpler, somewhere between 10 to 12 million barrels of oil cross the strait daily: approximately half that of pre-war volumes.
This varies significantly from day-to-day, however. On Sept 7, Mr Johnston said, 20 million barrels crossed the waterway, but by Sept 10, it had dropped to three.
Nevertheless, countries such as the UAE, which claims to be exporting two million barrels a day through the state oil company, Adnoc, have ramped up operations, and Kpler data show fuel volumes are at their highest since the start of the war.
Some, like Ms Ziemba, believe the clandestine operations are not a long-term solution but a “coping mechanism”.
Still, it is one with which Iran has failed to cope. It has failed to stem the increasing flow through the strait, prompting rumours that officials have been given pay-offs to turn a blind eye to the fuel smuggling.
Iran’s failure to clamp down on the shuttle boats may partially explain why its proxies “launched drones to attack the East-West pipeline” on Sept 10, forcing Saudi Arabia to reroute millions of barrels of oil, Mr Henderson said.
Fighting between the US and Iran has largely ground to a halt, barring the occasional skirmish. As long as the scale between risk and reward is tilted toward profit, the flow of fuel will continue.
What started out as a war almost seven months ago has turned into a numbers game.