What kind of thermal paste do I need for a heatbed thermistor?
If you need it to hold the thermistor in place, there are thermally conductive electrically insulating potting compounds, such as Appli-Thane® 7300 Urethane that have a temperature range of -100 to 160 °C.
The newest CPU thermal pastes contain silver particles and will conduct electricity. So, it needs to stay away from leads and pads that don't want to be shorted