Just to specify, oil Is leaking on the valve cover, NOT coolant.

 

Artskye.......Just to throw out a wild idea here.......it could be that the prior chap had thought it a "good idea" to pour a cup of some type of oil (Marvel Mystery Oil or tranny oil) into the radiator to kinda lube the water pump. Maybe the pump was chirping every once a while, and he thought this would solve the problem. Maybe just drive your rig, and if the tranny fluid continues to look good, with no antifreeze mixing with it, then maybe thats all it was.......someone adding some oil to the raidator to quiet the squeeks? Who knows........?????

 

You can send a sample of transmission fluid for analysis to confirm if it does or does not contain any coolant in it.

 

Blackstone labs is awesome, I had them do an oil analysis before I replaced my engine. Would highly recommend giving them a try.