From mid-September 2015 all E3D-v6 HotEnds will be supplied with a small sachet of thermal compound. This improves heat-transfer from the heat-break threads to the heat-sink for slightly better thermal performance in marginal cases. The thermal compound should be spread evenly across the threads of the heat-break, only on the cold-side of the heat-break that screws into the heat-sink. The compound should not be used on any of the threads on the hot-side of the heat-break. The small sachet of compound contains more than is needed for one HotEnd, so don't feel like you need to use all of it.