Here is my thought on this since it just happened to me while i was outside with it for a bit.
It seems that the issue is one of two things:
1. There is a vacuum effect going on. When the device heats up a vacuum is created between the digitizer and the lcd screen causing them to touch which in turn causes this "oil mark". The fact that it is happening in the middle, where there is no support, is a strong indication that this is happening.
2. There is something internally that is expanding due to the heat causing the lcd to be pushed outwards causing the two screens to touch. I doubt this is the cause but still could be a possibility.
I am confident that it is a vacuum that is being created between the two screens that is causing this. If so, this can be corrected. The Tab can be taken apart and using a small needle like a sewing needle, pierce some holes in the glue or double back tape that joins the digitizer and LCD together. This would allow air to come in and eliminate the vacuum. The smaller the holes, the less chance any type of dust will get in there.