Oil Replacement (changing from 10w40 to 5w40)

greetings friends



its my first topic on this forum so welcome me lol



i just want to hear opinions from u experienced guys.. i have 330ci (coupe) 2001y. previous user was using 10w40 (250k km run on it) and im thinking to change to 5w40 or 5w30..



what your thoughts are? will it make some impact on engine work? will this move improve it or vice versa?