No risk there as castrol is not synthetic although they claim it is so
highly modified its like synthetic .There was a court case where castrol won
saying their oil was synthetic but is based on mineral oil .Thats a bit like
saying well its almost synthetic .The Nazis developed synthetic oil for
their jet engines as normal oil was'nt up to the task .200 klm go for it
just change the filter al little earlier on the first change then as
normal.MY WIFE TURBO CAR HAS DONE @250 KLM MY SONS NORMALLY ASPERATED
AROUND 305 KLM AND MY BABY TURBO  @186 KLM no oil leak on any of them.
Around 113 kw at the rear wheels on a 1990 740 turbo says all is well .We
use 10-40 on mine and 5-50 on my wifes and my son gets the oil when we
finished with it .