2005 Silverado misfire & lean
 

First off just wanted to say thanks, been lurking here for a while and found some great info. Now for the problem.

2005 Silverado, Ext. Cab Z71, 5.3L, 131,000 miles. About a year ago while changing the spark plugs, I broke 1 (bank 2 closest to the firewall), so I put the original back in and neglected to replace it until a few months ago when the CEL came on. Truck started idling rough with a misfire occurring during take off, but once it warmed up seem to run fine. Had the codes checked, P0300 & P0174. Found a loose plug wire, cleaned MAF, and replaced the bank 2 O2 sensors (lots of carbon build up on them)and it ran like a champ for about a week, then started up again. I figured it was time for the other side. Yesterday I replaced the bank 1 O2 sensors even though they didn't look too bad, problem persists. Checked the plugs: bank 1 fine, bank 2 not so fine, This morning I replaced them and checked the O2 sensors, and once again lots of build up. Since they were new I used a can of compressed air to clean off the build up. It seemed to run a little better, but CEL came back on on the way to work. Had the DTC's read and got P0300, P0174, and P0151. I'm gonna try putting the sensor from Bank 1 in place of the one in bank 2 now that the new plugs are in and see if that helps before I throw more parts at it, but I feel like I'm just chasing my own tail.

Update-Swapped O2 sensor and Its running great again, but here's my new problem/question. When I pulled the old one there was a lot of oil on the outside and the inside of the connector. It was like this the first time I changed it too. I cleaned the contacts and used some electric connector grease to try and keep it from happening again. Anybody ever seen this before or know what might have caused it?