Oil is an insanely complicated subject!
As well as viscosity (the 10/40 or 0/60 number), there's the API S letter, and other considerations such as "cling" and how it treats seals.
Semi-synthetic tends to have more "cling" than fully synthetic and so hangs around the top end of your engine for longer - 10/40 Selenia semi-synthetic lubricated cars seem to have fewer problems with variators than fully synth cars.
Fully synthetic tends to do funny things to your seals leading to leaks.
One recent example was an owner who wanted a lot of work done to his Alfa, including a flywheel end crank oil seal leak - not the easiest of jobs. As it was going to take a few days to do all the work, it was pencilled in for when he was on his summer holiday. As part of the discussion we chatted about oil for a bit and I recommended 10/40 semi-synth.
When he came back to me last month, he said he didn't need the oil leak dealt with any more as switching to the semi-synth for three months cured the leak...
All I can say in response to other opinions is that I see several 156s a week. That I have seen the same cars in for servicing again and again as they pile up the miles. And that main dealers see very few cars once they have done 36K miles or so.
All my opinions are based on a large sample of observed effects and patterns of failure of cars not necessarily in the first flush of youth.
Roberts Aerospace and Automotive
PS. I always use fully synthetic oil in my Karif 2 engined 290bhp Maserati 222E, but that's a very different car...