You don't get it, do you?
A nuclear was is existential.
The survivors will make the decisions on where they'll buy oil from and it'll likely be from whoever has it for sale.
That is only true if both combatants have substantial nuclear arsenals. The US attacks on Hiroshima and Nagasaki did not threaten the existence of the United States, because Japan lacked nuclear weapons. A Russian nuclear strike on Ukraine would in similar fashion not threaten the existence of Russia - unless some 'third party' nuclear power volunteered to risk it's very existence on behalf of Ukraine.
People assume the US is such a potentially suicidal third party, even though we are not obligated by any treaty to intervene, and Ukrainian independence is not a vital US interest. Would the US intervene with nuclear forces if India nuked Bangladesh? Or if Pakistan nuked Afghanistan?
Talk is cheap (especially when the speaker is senile), and hopefully there are enough sentient adults left in DC to prevent national suicide over a war that most Americans can't even find on a map...
;>)