to say it is infinite isn't really true. we are using it up quickly but it is always reproducing itself , it just takes thousands of year to happen. i'm guessing if the truth was to be told, we have a lot more then the oil companies want to admit.
It's a pretty well understood fact that there's not that much oil left that is "Easily and Economically Recoverable" that is the big problem - finding it and then getting it. Wikipedia has done a pretty good job of explaining Peak Oil and has some interesting quotes from top oil executives discussing the topic and in particular the relationship between "finding it" and "getting it" at an economical price.