The NTE303A is a non−reactive, silicone, thermally conductive grease with a high thermal conductivity and low thermal resistance with a soft, non−flowable consistency. The product is formulated with specialty binding agents to achieve the lowest amount of bleed and evaporations. It is designed for applications where a silicone thermal interface material is required and where the device may later need to be easily remove from the heat sink.
 
Full spec sheet:
https://www.nteinc.com/specs/300to399/pdf/nte303a.pdf