Ok stupid question but there seems to be a fair difference between 5w 40 - 15w 40. What advantages do each of them have over disadvantages? Or is it just preference? I have 170k kms on clock & last service was 166k so im thinking do/ should it be serviced every 5k kms or 10k kms? I have heard mixed opinions so far! Ps Just bought it 2006 GU IV ST!