I want to put a strong magnet on my gearbox drain plug and engine oil drain plug on the outside where the nut is the bolts are 12mm long has to be a very strong magnet to pick up any grains of metal floating around inside and it has to keep its magnetic force when engine gets hot could you please advice me which type would suit?

Many people use Ferrite Magnets for this application. They can withstand about 250 degree temp. People also use Samarium Cobalt Magnets or Alnico Magnets because of their high resistance to demagnetisation