oil viscosity grade

Hi, Hope this isn't a daft question, but I see from the handbook that there is more than one viscosity of oil suitable for my CLK 270 CDI. I happen to have 10 litres of 5w30 that meets the correct M-B spec, but it's in the back of my mind that maybe after 100k miles I should be using a higher...

Advice please as about to do an oil change, Castrol recommend Edge 5/30 or 0/40 Fully synthetic oil for an A150 W169 (its done 53,000 miles) (Mobil 1 not available in my location) It presently has Edge 0/40, is that the best choice of the two for my circumstances or doesnt it matter...