You sure you put the lambda's back on the same way as they came off?? heard of similar problems in the past after people have replaced these and its cuase they've put them back on the wrong way round. The could explain the overfuelling and the misfires, doesn't explain the header tank/oil filler symptons they do point to h gasket. I'd def be tempted to swap the lambda's round as it only takes 5 mins to see if it makes any difference.