For 400I Auto year 1984 DOT3 or DOT4 ?
DOT 3 is original fluid, don't mix brake fluids, you could use DOT 4 if you are completely cleaning/flushing the entire brake system, 4 has a higher boiling point. Gary PS: Use the "search" function on top of page, click "more" to enter key words & search only this forum. Lots of good info here.
No problem mixing DOT 3 & 4 as they are both glycol based. Boiling point of a mixture would be tough to calculate but would ostensibly be an improvement over 100% DOT3. It's DOT 5 silicone fluid that shouldn't be mixed with others. If going to DOT 5 which is not hygroscopic thoroughly flush all the old fluids first. Dot 5 should only go into a dry system which is going to require some dismantling.
You can use DOT5.1 which is also a glycol based fluid with higher boiling point than DOT3 and DOT4. I would never use DOT 5 in a glycol fluid based brake system because there i s danger of destroying the rubber parts. Results are clear if this happens.
