[barrkel]

The appearance of oil on the surface comes from roasting beans to the second crack:

https://mtpak.coffee/coffee-roasting-basics-guide-to-first-s...

Oily beans are dark roasted and have a different flavour profile. If you get cheap supermarket espresso beans, they're usually roasted to this point. I think it gives a more consistent product given input variability, varietal differences are no longer so apparent.