This is from the slave I removed years ago and I recently found the exact same deposits in the clutch master cylinder when I replaced it. Light grey in colour, appears granular but when squeezed between fingers it is a smooth paste but, strangely enough, not greasy at all. If it is assembly lube there sure is a lot of it.
The slave is the factory original and the m/c which I replaced recently was a five year old Sachs I installed in 2012. I replace the fluid every couple of years, using the called for DOT 3. The old fluid always appears to be in pretty good condition without much discolouration.