Some super-automatic espresso machines with an internal grinder recommend not using very oily coffee beans. While our Espresso Blend Whole Bean is roasted darker for a bolder espresso taste, more oil is released from the Dark Roast. The oil is actually a result of the roast level and not the origin (longer roasts release more oil on the surface of the beans). With this in mind, a Medium Roast may be better for fully automatic coffee and espresso makers with built-in coffee grinders.
One other thing to note: oil on the surface of beans can either be due to a Dark Roast or because they are very old. Age and exposure to oxygen over time pull lipids out of all coffee beans, regardless of roast level. So your older, stale coffee beans will become more oily than fresh coffee beans. However, darker roasts get oily almost immediately after they are roasted, but this does not mean they are stale or old.