its a 05 ford focus 1.6zetec heard 10/40 is bad for them.
cheers andy

 

some say 5w/30,imo its down to how the engine has been treated.im the second owner on my focus and the person that owned before me was a women who only did milk miles.ive been using castrol magnatec 10/40 for the past 3yrs and never had a problem.

 

All modern petrol engined fords are designed to be run on 5/30 mate

You can use 10/40, however I wouldn't recommend it as it will accelerate engine wear.