Any lubricant that makes the filament glide easier in the flexible guide tube leading to the extruder and also inside the extruder where the filament travels down to the hot-end where it melts.
We have made many tests with common mineral oil (for sewing machines, bicycle chains, etc) which works well. We have also tried with cooking oil which had the same positive effect.
As filament we use almost every polymer which we can find out there such as PLA, ABS, Nylon, etc. (we love to experiment) :)