Took apart an engine that was recently bought. Rotor sides are covered with what appears to be dried up oil residue.
Side seals are being held in by this residue really, really good!

Any suggestions? Should I submerge the rotor completely in some chemical?

 

I usually use kerosene or diesel. Submerge it and just let it soak for a while (days, if you can), then occasionally agitate the seals by gently prying on them. Keep the mindset that you don't have to get the seal out in one shot, but also be one with possibility of needing to replace all 12.