Liquid preloader

I am trying to make a preloader that has some cool liquid effects. I want it to look like a gas tank being filled up, viewed from the side. I am trying to script it so that the surface of the fuel sloshes around as it fills up the tank, but I just can’t figure out a good way to go at it. Can anybody offer me some suggestions on how to achieve this with actionscripting? Maybe you have seen a similar effect somewhere that I could check out?