As far as I know nobody has had an engine failure due to using the wrong brand of oil. Of course Porsche owners don't want good for their babies, they want best, let the hijinx begin.
Mobil 1, what I used for years, supposedly formulation changed this year.
Some people are having low oil pressure warning lights come on with the new very low viscosity low friction oils. Long discussions are around related to this, summary is that it isn't clear what the low pressure warning means except that 928 motors are 20+ years old and not put together as tight as modern engines. Oil pressure isn't what protects the motor, flow and film strength protect the motor, but low enough pressure and you get lifter noise, so nobody likes it.
Some have switched back to a higher viscosity oil, but some are also looking into the internal oil pressure regulation system to see if problems with it exist.
Any engine the leaks as much as a typical 928 should run synthetic since it stinks a lot less.
I now use Rotella T synthetic from walmart, but its just oil.
Confusion about oil, oil specifications, how oil works, what the numbers mean, formulation, generally complicates a discussion among hobbyist users like ourselves. I'm not sure its worth the effort to drift away from 15w50, YMMV.