I realize different country have different maintenance schedules for the Mirage. We have a 7,500 mile (12,000 km) & 3,750 mile (6,000 km) recommended oil/filter change interval (depending on which schedule you are following) in the States.
20,000 km = approximately 12,500 miles
I realize synthetic oils are quite good these days, but I like changing my oil/filter every 5,000 miles. I sort of feel that a clean oil/filter is more important than the actual brands used. I do think there are a few superior brands, but if your oil is changed more frequently I don't think it matters that much.