for the first time i used a fully synthetic oil just to see what the fuss was all about.it was millers oil 10-60.what i found was that when driven hard for long periods the engine sound was a lot louder when i stopped on tickover and the fan would come on alot sooner. now this never happened with the semi syn i was using .does this mean that the oil was o thin?i did 3600 miles on this 10-60 and i must say that when the oil was removed it was still in good condition.ive gone back to semi syn again i must say that the engine seems quieter.the technical department at subaru uk recommend standard mineral oil. i have spoken to someone in that department last week he just kept on saying avoid synthetic at all costs ,i have the facts to prove he said.semi alright 15-40 or 10-40 . this is not a big end thread ! i would like to see tables on varius oils if there is one.has anyonelse noticed the diffrence between mineral semi and synthetic.