El Saud was put in place as rulers of Saudi Arabia by the British, not the Americans.

Arguably still oppressive after we’re done but better than before.

I suspect what happened is not the history you think it is.

I hate justifying the US's behavior in this regard, but all such critiques need to at least be context-aware.

The USSR was attempting to spread a system of totalitarian social, economic and thought control world wide. At the time, almost everything by comparison was a lesser of two evils. In opposing their efforts, anyone who also opposed the USSR, be it a European Democracy or a third world strongman, became a US ally. Anything to thwart the spread of the USSR's variant of Communism. Supporting strongmen was ugly and unpleasant, but those were also desperate times.

The USSR was expanding rapidly and adding satellite states left and right. Any strongman who 1) opposed the USSR, and 2) who was strong enough to maintain control of their country despite USSR attempts to destabilize and gain proxy control of it, was a potential valuable ally in the greater contest. Sometimes the strongmen were the only ones in a given country who could meet both criteria, and the US had to work with was available. Ugly, but c'est realpolitik.

Though I'm not sure Iran can be justified even under that framework. Fomenting a coup in a country with a democratically elected government and liberalizing society just so we can take their oil instead of buying it at market rates is unjustifiable, even in a Cold War context. Their oil could potentially tip the military balance of power, but surely an alternative to a coup was some kind of oil-for-military-aid treaty with a fellow Democracy.