This is to some extant based of the premise that the US intervened in Iran due to British Oil interests.
One would assume that if that behavior was consistent that the US would also have intervened in Saudi Arabia when Aramco was nationalized.
Is the premise incorrect? Or were there other deciding factors?
I don't know much about the Aramco case, but I'd like to challenge your assumption that the behavior would have to be consistent with the Iran situation. In 1938, when President Cardenas expropriated foreign oil assets in Mexico, including many American assets, the US government under Roosevelt did not intervene. This was much to the chagrin of the oil companies, who felt betrayed.
Fun quotes (all from Yergin's The Prize, emphasis added):
"Though Mexican Eagle, a British company, was by far the largest producer, much of the agitation against the oil companies was based upon the strong anti-United States sentiment that seemed to unite the country. "The one respect in which I have found Mexicans of all classes completely unanimous," observed an English diplomat, "is their conviction that it is a fixed principle of American policy to prevent the economic development and political consolidation of their country." Yet, ironically, the diplomatic support that the American companies had previously counted on was now a thing of the past. The Roosevelt Administration had adopted a "Good Neighbor" policy toward Latin America, and the New Deal viewed the Mexican government's stance with some sympathy. From a foreign policy point of view, Washington was keen to avoid alienating Mexico at a time when concern for hemispheric defense and fear of an impending war were both beginning to mount. Thus, there was little pressure from the North to counterbalance the unions' radical demands.
The crisis deepened when the Mexican Supreme Court upheld the judgment against the companies. The companies, in turn, upped their wage offers twice.
Then came March 18, now celebrated as a public holiday in Mexico:
At 9:45 PM, he signed the expropriation order, and then broadcast the momentous news to the nation from the Yellow Room in his presidential palace. His words were greeted with a six-hour parade through Mexico City...To the companies, the expropriation was absolutely illegitimate.
Since most of the assets were British, how did the British government respond?
The British government took a very strong stand against Mexico. It insisted that the properties be returned. Instead of replying, Mexico severed diplomatic relations.