The brake fluid is separate from the oil in the differential. The spec calls for Dexron fluid in the master cylinder reservoir. This fluid is contained in the brake piston by seals. These seals are what gets damaged when DOT3 is added to the brake reservoir. The volume of oil in the brake system is separate from the differential. The damage done to the brake discs occurred over time. The Dexron fluid will mix with the JD20 without causing great damage.