I too always use Castrol XL30 and have had no problems. My reason was not so precise as Phillippe, as I had heard a horror story from Mark Harding who had rebuilt an engine at some cost and the customer had filled it with Mobil 1 synthetic oil! 500 miles later the detergent had done its stuff and removed 70 years of baked crud and with no filter had run it through the bearings etc. I decided to follow the owners manual! Castrol XL30 for ever more!
Julian Pratt