I think the word "synthetic" might be a bit misleading. DOT3 and DOT4 are glycol-based fluids, and DOT5 is silicon-based. DOT3/4 absorbs water, DOT5 does not which makes it good for brake systems that sit out in the weather year round or are stored where it may be cool/cold and damp. The difference between DOT3 and DOT4 is the boiling point. DOT4 has borate esters added to increase the boiling point.
There are pluses and minuses to DOT5. On the plus side, of course, is it does not absorb moisture and it's boiling point is higher. On the minus side is it's much more difficult to get the bubbles out so it's very important to be very careful when adding the fluid to the MC that you not create bubbles when you pour. And for heavens sake don't let the MC run dry while you're bleeding the brakes! Another minus is that the hotter the brakes get, and subsequently the brake fluid, the spongier silicone fluid brakes will feel. Another minus is that if ant moisture does get into the system it stays as pure water and does not mix with the silicone therefore it can lower the overall boiling point and can create rust pockets within the system.
The best time to convert to DOT5 is when you are replacing the entire braking system. You can not mix DOT5 with any other version. If you have an existing system and want to convert there is a way but it's a lot of work and involves flushing the entire system, usually with alcohol.
Generally speaking DOT4 is more than good enough for standard flat fender applications.
Larry
