I have recently been using Shell VSX Synthetic. I have also recently been having problems with poor running when cold, progressiveky getting worse and only fixed by fitting a new spark plug each time. I cvannot be sure that its the new opil but that is the only think I have changed recently. I am going to try a hotter plug.
Sorry this does not diectly answer your question Griffo but hopefully others can chip in and at least we can rule out some unsuitable oils