From my research and other people's practice 3 and 4 can be safely replaced with 5.1. My other bike had DOT3 for clutch hydraulics, and DOT4 for brakes. DOT5.1 is compatible with both and I had absolutely no problem flushing both systems without cleaning and drying them. So, I am sure now the modern 5.1 can be safely used on KZ.
In my case, the whole hydraulic system has been disassembled and cleaned, so there is now issue.
