The US does not merely buy oil from the Saudis. It lavishes them with the weapons and surveillance tech they use to destroy Yemen and crush domestic dissent. One of the overlooked Snowden stories was how Obama ordered NSA to share more with Saudi tyrants:
https://theintercept.com/2014/07/25/nsas-new-partner-spying-saudi-arabias-brutal-state-police/
https://twitter.com/ggreenwald/status/1548005160110006273