850 ('97) Oil types

I've got an 850 ('97) basic model, I was wondering, what type of oil would give my engine the best protection? It currently has 10w 40 and gets used for short motorway and town driving. Would a thinner (5w 30) fully synthetic oil give more protection to my engine? It has done just under 180K miles. Any input would be really appreciated. Dodski.

10W40 SEMI certainly wouldnt go thinner especially on a higher mileage car

As above, Mine's on 196k and I use carlube cheap 10/40 semi and change it every 5-6k. Still silky smooth :D