Agree - have just ordered my oils from Opie Oils - 5 litres about £26 plus delivery. If you go to Opie site and enter car details it will come up on recommended list.Fuchs titan Supersyn 5w40 is what you need
I would do what Tony says and try an engine flush, but if that does not work then I would replace the lifters, gunge just grows no matter how much you thin the oil out, and then you'll be into head problemsOn start up its not firing on all four for about 30 seconds