I've played every version of Civ since the first one. Just got around to Civ4 and so far have played two games. In each, I was cruising along and had identified a civilization that was clearly In The Way. However, it was not practical to attack them because while I had superior tech, I couldn't make use of it because the only oil on the continent was deep in their territory. They didn't offer it for trade (perhaps hadn't discovered the necessary techs yet) and the only other oil was out on platforms (which require plastics...hundreds of turns of research away). A lot of the tech tree requires oil to unlock, yet it does not seem particularly abundant (in the two games I've played, granted not much of a statistical sample). Looks to me like the entire game would revolve around a few squares of oil. Without it, you're stuck, as to me it doesn't seem like you can get enough of a tech edge in the pre-industrial techs to make assaults on well-defended cities worthwhile. More to the point, I've always found in Civ (going back to the first one), that any large war requires Railroad, otherwise you get bogged down in logistics.