Trans fluid and rear end fluid
#2
trans fluid depends on type of fluid mileage and severity of service. a stock gm trans with the dexron 6 fluid and reasonable treatment should go 100k miles on the factory fill. An enthusiasts car I would recomend no more than 40k miles between service as we enthusiasts tend to be a little less than reasonable lol. On the rear end roughly the same
#6
I don't believe a M6 is nearly as hard on fluid as the A4. But for as little fluid as it takes, you might as well change it. Plus you could go with something like Honda ATF for super smooth operation. The stuff is pricey but you only need 5 or 6 quarts.