C402 pilot reported a loss of engine power and engine roughness resulted in a return to the departure airport.

Narrative

Takeoff and climb out of ZZZ was normal with normal engine indications. About 10-15 minutes into the flight as we leveled off in cruise. Both the First Officer and I noticed a significant vibration. We did not see anything different on the engine instruments nor could we see anything to indicate where the vibration was coming from out the window. A few minutes later we started to notice a fluctuation on the manifold pressure gauge. We noticed that the right manifold pressure would fluctuate plus or minus 5 inches -10 inches of manifold pressure. It would then stabilize and then fluctuate again. We then decided to return to ZZZ and requested priority from ATC. On our way back to ZZZ we consulted the QRH for anything that could help with a rough running engine. We couldn't find anything; so we just left the QRH open to the Engine Failure During Flight page. The vibration never stopped; and the manifold pressure dropped to as low as 23 inches with the throttle full forward. We were able to safely land the plane and taxi in without incident. The passengers were informed and taken back upstairs. Once on the ground; we were able to take a look at the right engine and noticed the entire bottom of the engine nacelle behind the cowl flap was covered in oil.