Are areas filled with an oily texture common in jamon serrano
As I don't have the necessary points to comment I will make one or two assumptions.

Spanish Jamon is a salt cured ham. I suspect, if you got your ham bone-out, then the residue oil is where the meat closest to where the bone was did not cure as well. 

Make sure you buy your Jamon ham from a reliable and recognized source (Denominación de Origen). Even if you saved a little on price, with food, it is not the greatest plan.

As I live in the US I only enjoy that delicacy when I travel to Spain. I do not trust any of the US imports.

You just haven't lived until you have tried Jamón de Guijuelo!!!