Engine Oil for 2.5 TDI: Best Choice & Guide

Hey everyone!
'I need to change the oil in my Audi A6 2.5 TDI. According to the previous owner, it currently has Castrol SLX 0W-30 (full synthetic) in it. What oil is best for the engine, as some friends advise against using Castrol? Can I switch from full synthetic to a semi-synthetic oil now?'
A friend has been using EUROL 10-40 and raves about it. Should I use it?

Best regards,
saxi

Check this out: 'Tips & Tricks for Care and Crafts' is in the menu on the left.

See oil viscosities and grades.

Best regards, Rainer.