The manual and Bentley guide recommend 10W30 over 5W30 oils(2002V6PassatAuto). They really make it seem that there is a real big difference. I could not find 10W30 at my auto store tthe other day so I used 5W30 Castrol . Is there a big difference and can it cause a big problem? I am thinking about changing to Synthetics but I don't think I want to risk the leakage thing. Thanks.