Thanks,
This is a very good hint for the syntetic use for me.

About marks/mirror look - would one expect original mirror finish to stay as good as new if there is no dirt???
Or is it normal for finish to go from extremly clear mirror surface to kind of a bit foggier mirror (still mirror you can look into, but you know - not as perfectly poslished as a good glass mirror but more like a polished bit of metal or tin???

As for marks - thanks I will take the engine apart and clean it before running again, as I suspect this may be the case..Although I can't see were dirt came from as I had muffler on and Norvel has got an air filter on the venturi.