In light of the description of that lubricant, I'm a bit surprised on using it on this type machine. Are you certain it is used based on the gearbox OEM recommendations?
All RENOLIN DTA products are DIN 51524-1 (HL) hydraulic oils and DIN 51517-2 (CL) general lubricating oils, based on mineral oils, demul-sifying (water-repellent) and free of zinc.
One of the best references on high speed gearing lubrication, titled Cooling and Lubrication of High-Speed Helical Gears can be found at http://turbolab.tamu.edu/proc/...oproc/T33/t33-06.pdf
. If you seek out "axial meshing velocity" on page 2 by Acrobat reader (42 as the article is numbered) you will find the reason for the burnt coloring all the way toward one end of the single helical gearing.
Beyond what you learn there I would do an internal inspection of the gearing through an inspection cover and make sure that a spray nozzle hasn't become plugged. Check with the OEM or your lubrication vendor and see what methods you might be able to use to clean off the varnish through the inspection opening. Then verify you are using a recommended lubricant.
You might ask yourself as well about the possibility of varnish on bearing surfaces. Do you have a history or sudden onset of elevated bearing temperatures?