I know this has been an inactive thread for a little while... I just bought a 1DX recently and I'm having some of these oil problems too it seems. The sixth digit is a 9 on mine. It has no markings. Came with 1.2.4 installed. I checked the sensor almost right away when I bought it and it was essentially perfect; couple little dust specs but that's all. There were 3 very light specs in my viewfinder (1 top left, 1 bottom left, 1 top right.) After around 500 shots or so my sensor was becoming quite dirty!! I hadn't changed the lens a single time either. I decided to give it a wet clean. Took 5 pads and was able to get it almost spotless. I was also able to remove the top left and bottom left viewfinder specs by gently using a rocket blower. However, the top right wouldn't budge. Anyways, I've taken around another 500 shots and my sensor is already showing quite a bit of oil. But what really ticks me off is the fact that I just checked my viewfinder again and now there's 3 specs in the top right corner! Seems like mine, with a 9 in the serial, has the oil problem. What a PITA.