In my experience, it has little to do with your rice cooker model, the amount of water (so long as it's reasonable i.e. not extremely scarce), or adding oil whatsoever (frankly, greasy white rice sounds pretty disgusting to me).
As a red-blooded Asian who loves rice and have been cooking it all my life, in various rice cooker models, using various types of rice, I used to have this problem when I was a wee beginner but eventually figured out a method to eliminate that stubborn crust:
The key is to let your rice rest for awhile after it's cooked. When the rice cooker tab pops up to indicate your rice is done, do not immediately open the lid. Instead, let it simmer down for a minute or two, allowing all those boiled water bubbles to settle.
Following this, open the lid (I usually pop the cable off the socket for good measure) and let the rice steam out until cooled to your taste. I personally leave it for five to ten minutes, and that results in just the right level of rice warmth for me, which is on the cooler side. Your mileage may vary so test this out with different times to find a temperature that suits your liking. And If you want to reheat the rice, it should be fine by now to simply pop the setting back on to "warm".
Voila--no more crusting on the pan. Take into account that rice is sticky by nature, and a small amount of sticking on the pan is inevitable, but this should work to reduce that overall layer of crusting.
This is my way to prevent a hard crust on the bottom of the rice cooker pan. I'm sure others have other ways that have been proven to work, but this one works great for me.
Note: this works for white rice in general, which is the most common and the type I'm assuming the asker was talking about. Other types such as red or black rice would require more water and tend to crust more around the top, I have yet to successfully handle those.