Well, Mobil 1 is pretty good. Fully synthetic motor oil is what I use too but I use Shell Helix Ulta 5W-40. 20,000Kms is a long time for a oil change. Even Shell recommends you to change your engine oil at 10,000Kms, but I won't risk it. I think you wouldn't too. In my own opinion the best mileage to change your engine oil is at 3,500-4,000Kms.