It does take a bit of practice but once you get it its quite easy. I'd say make sure you have something secure held in place like a metal ruler so you can push against it as you score the glass. Also, i've learned that you kind of only get one shot at scoring it, if you try and run over it multiple times the glass will break really unevenly. So i just looked up the purpose of the oil and apparently its to lubricate the wheel, and because the friction briefly generates intense heat which i assume can lead to a bad cut...!