There always was a difference between the interests of capital-in-general and those of the individual capitalists. The 19c. imperialists always understood that prestige was quite as tangible a thing as capital. There was a lot of evidence cited that the US capitalists were surprised by the money they made during the Spanish-American war, which tempted them into supporting imperialism.
As for war-for-oil, this always was the reductio ad absurdem of radical politics - the left's commodity fetishism, investing dead objects like oil with the living power to change the course of events. So much easier to 'follow the money' (or at least to impute a monetary motive without thinking it through) than to theoretically grasp the way that the ruling class came to its decisions.
"It makes me wonder who really drove all that inter-imperialist war in the 19th and early 20th centuries. My pal who's doing a diss on the City of London says the City didn't want WW I - the politicians did. The City was making too much money floating securities for Germany. More recently, I don't think it was even the oil industry that pushed for the invasion of Iraq; that came from politicians and right-wing intellectuals. Now they may have a "better" grasp of the long-term interest of capital than the capitalists themselves, but a lot of capitalists just prefer the status quo to something as risky as war."