oil in desiccant cartridge

Hi, Im new to this forum and am looking for some info on our 2012 international 4400 air brake system. We had it come in the shop the other day with a complaint the purge valve was sticking open a bit longer than they normally noticed. So on inspection of the air dryer system I noticed a bit of oil residue around the valve. We decided to change out the desiccant cartridge and pick up a rebuild kit for the purge valve. Once I pulled the cartridge I noticed quite a bit of oil residue inside of it. Im curious if this is normal or if it should be dry inside? I contacted our local international dealer and they thought it should be dry inside. So on that note I have been researching and trying to find any and all info on these systems along with info on the air pump or diagrams of break downs of the air pump to see if there is an issue there as well, bad seals or what not that might cause the oil to bleed into the air lines into the dryer. Or if this is just normal residual oil and the cartridge was just due for a new one? Any and all info would be greatly appreciated, thank you.

And also this particular truck is a durastar DT466 with only 58000 on it.

 

There wasn't any residue in the tanks but im suspecting we might have caught it before it got any worse. There was a little residue in the lines just out of the compressor when I pulled the lines to check so was thinking the seals were bad or on their way out, but just wanted to get a few different opinions. I appreciate it.