I would argue oil was also secondary. The primary reason was to make an example out of a country that was becoming a little too independent. It was supposed to be the first domino in a longer line of countries in the region left over from the cold war that were not yet aligned with the US (or at least not sufficiently integrated into the neoliberal hegemonic world order) and needed their governments toppled one way or another. The US wanted to turn the entire Middle East into obedient client states.