Thermal pads should be replaced with thermal pads only or with specially designed viscous thermal paste. The reason is that normal thermal paste is not designed to fill big gaps so sooner or latter will flow out and leave a gap between the component and the heat sink. Personally I always use K5 PRO thermal paste for thermal pad replacement. It is very viscous and with high thermal conductivity and also specially designed for this reason. It costs around 5$ for 20g on Amazon and ebay (including free shipping worldwide!!!)
http://www.ebay.com/itm/390949819525