Use BRAKE fluid to Specification DOT 4. The time allowance for BRAKE fluid service is 70 time units (around 3/4 of an hour) and would use 2 litres of fluid (£6.64 a litre) as it is flushed as opposed to just changed. There is a set procedure for BRAKE fluid service on S3 and if not followed you will end up with very spongey BRAKES and clutch. Useful reading here
Audi Brake Fluid - ATE Audi Brake Fluid - Audi Brake Flush