You got me thinking so I did some Internet searches. For the most part I stayed away from opinions on forums. I did find three references that sounded like they knew what they were talking about
1. DOT 5 Brake Fluids
on Greater Atlanta British Motorcycle Association website
2. Selecting Brake Fluid
on Buckeye Triumphs website
3. Silicone-based DOT5: busting the myth
on (of all things) Mountain Bike Review's website.
The last one was the only forum I referenced but I compared all three to the information found on EBC Brake's
and Brake Tech's
websites. Both of the brake websites seemed to confirm the other sites information but just didn't explain it as well.
DOT 3, 4 and 5.1 are glycol-based fluids and play nice with each other. DOT 5 is silicone based and doesn't play nice with the others.
The main advantages of DOT 5 seem to be it doesn't destroy paint and it doesn't absorb water. These traits make it ideal for classic vehicles that aren't used too much since you don't worry about damaging the paint and you don't have to change it out every 2-5 years. Also it has superior lubrication of master cylinder and caliper pistons which could result in nearly zero component wear.
- It is more susceptible to air bubbles which will reduce performance
- It is less compressable so it doesn't work in ABS systems and may feel softer
- Allows water to pool in the calipers which could cause corrosion
- A lot more expensive
- Has negative effects if used in a DOT 3/4/5.1 systems that wasn't properly purged.
Every single reference I could find agree that DOT 5 will NOT harm the brake system seals or any other rubber parts.
Personally I'll stick with DOT 4 as recommended in the owners manual.