Can we see a Pic. of the outside of the transformer. A side shot might do some justice to see what is going on. Most transformers that I have seen have a thermal pattern of hot at the top and cool toward the bottom of the fins. The thermal should be uniformly through out the fins. Some times if a transformer is not level you can see an uneven pattern in the fins. But if you have low oil in the transformer you should see the same pattern on the side of it. I would concur with looking at the thermal you have attached I would say the oil is low. But a shot of the side would be good. Keep us posted.
Regards
Jeremy Kays Reliability Technician
Level II IR, MCA Level I, In Compliance with the ASNT guidelines
The Snell Group
jkays@thesnellgroup.com
http://www.thesnellgroup.com