The previous owner was using Unocal 20/50 Synthetic oil. The engine has 103K miles on it and runs great. I am thinking about switching over to either Mobil 1 or Castrol 10/40 Synthetic oil. Is this a good weight oil for the V12? I live in the San Francisco, CA area. Unocal 20/50 synthetic oil is not easy to find. Your recommendations are welcome.