For the castrol people it was mentioned a while back by opies oil it wasn't too great once at running temps as other oils were hence more people use other brands. I used mobi 1 15/50 till they changed it to 10/60. Did an oil change the other day and have to agree with chrisa it did seem to rattle a bit to start but did clear up and hasn't seemed to since. Plus the fact it's available from every halfords is why I prefer using it!!