Michelin Primacy is a good motorway tyre. I used to get 36K from a set on my old 3.7 on the v1 Pilot Primacy. They're up to Primacy 4 now. I see in their advertising that they claim they are 'safe when new, safe when worn' and that matches my experience too. Compare to Pirelli PZero which were epic when new and tried to kill you after they were about half worn
I run Goodyear Eagle F1 on the S8 but that only does 5K a year so grip and performance are more important than longevity. You'd probably need two sets a year of those!
Kumho Ecsta is a good midrange choice for motorway miles. Less than half the price of Michelins for a bit more than half the mileage.