More specifically: oil + ability and political willingness to exploit it yourself and share the profits among all citizens.

As opposed to selling exploitation rights to investors to enrich a kleptocracy.

I have an impression that they're are saving oil/profits for some possible "black day" in the future. And, the "profits" which are shared among all citizens come from high taxes. I might be [partially] wrong though.

Well, a large part of the oil revenues are put into an investment fund because the oil isn't going to last forever. Profits of that fund go into the public budget, though.

According to Wikipedia, there are discussions about how much of the oil revenues should be saved; opponents of using more of them directly say that it would just lead to inflation.

https://en.wikipedia.org/wiki/The_Government_Pension_Fund_of...

No, you are correct. The oil money is hardly spent at all. It is invested for future security.