I wonder how long a good synthetic motor oil (like Castrol 0W30 or Mobil 1) can be used after its manufacturing date ? I just bought a couple of German Castrol 0W30 quarts with the SL designation. I wonder how old this is. Does anyone know how to interpret the numbering codes on the bottles to figure out a Castrol or Mobil 1 manufacturing date ? Any input would be appreciated.