wait a min, dont you get that problem of high temps when you use too much thermal paste
the guy in shop sounds like a noob at what he does,
i know someone that got a job at thoes places because he could put a motherboard,cpu,cooler,ram and hdd in, in under 30mins, sorry
go to a reputable shop next time, pc world are generally c***
and most thermal paste that comes with the cooler is grey and the cpu would probably fry before the thermal paste