Engine oil! Wich one should I put in?

[Lipinhu]

Hi
I´m doing the first maintenance in a few days, so new fluids need to go in.
The issue is that my engine goes with 320k Km´s. When driving the car everithing looks good, the engine rev´s good, no smoking....
I mean, with that distance driven the engine should be a little weared, there should be some gaps, but i dont feel any of it when driving.

But, in an engine that has done that far is wise to use a fully syntetic oil? 0w, 5w? Or should i go with a semi-syntetic? 10w?