Re: Which Selenia Oil should I use for a new 156
Pick a fully sypathetic oil with the first rating of 0W or 5W, and the second rating as specified in the handbook. Also, check the API rating on the oil, it should be something like 'API SJ' (which is the rating on Selenia 20K). Then, as long as you get an oil with a better API rating (which, after all, is what the oils are officially tested and graded against) then you can't go far wrong - unless this puts your vehicle out of warrenty on a technical basis, even though you are using a better quality product.
Lots of people think Selenia 20K is the best for the Twin Spark engines because that is what is recommend in the handbook. The reality is that at the time of engine design (probably late 80's or early 90's) Selenia 20K probably was one of the best oils on the market. This is no longer the case, and the API SJ rating has been now superceded buy another 2 or 3 better testing standards.
Ultimately though, regardless of what people think is best, if you stick with a decent(ish) oil with the same specifications as those recommended in the handbook, and also ensure that the oil level is always just below maximum, and regularly change the oil say between 6000 and 10,000 miles intervals (depending on driving and the colour of the oil) then you will be O.K.