The key thing to remember when comparing DOT 3 & 4 fluids is the formulation of those fluids. While all that has been said in this thread about the Glycol base of the fluids out there being true for all Dot 3 & 4 fluids, it is the specific formulation of the fluids that causes them to be considered Synthetic. In this particular use of the word Synthetic, it is used to describe the superior performance characteristics of the fluid when compared to the minimum DOT requirements set forth for brake fluid qualifications.
The minimum ("dry": meaning not containing water contamination; "wet": containing water contamination) boiling point according to Federal DOT standards for DOT 3 fluid is "DRY" = 205°C (401°F) : "WET" = 140°C (284°F) and for DOT 4 is DRY 230°C (446°F) : WET 155°C (311°F) and DOT 5 is DRY 260ºC(500º F) : WET 180º(356º F).
When comparing "Synthetic" brake fluids, you must not only look at the fact that they have a Glycol base, but also take into consideration the other ingredient formulation combined with the rated boiling points of those brake fluids.
Amsoil Dot 3 brake fluid has a minimum boiling point of DRY 274°C (525°F) : WET 156°C (313°F) and their DOT 4 fluid has a minimum boiling point of DRY 304°C (580°F) : WET 210°C (410°F).
Amsoil "Synthetic" brake fluid formulation minimum boiling point surpasses the minimum DOT boiling point requirements of even DOT 5 fluid requirements without being a Silicone based fluid. This ensures less viscosity break down of the fluid due to condensation build up in the braking system which means that the fluid is better able to lubricate and protect the braking system better than other brake fluids available.
Yes, using the word synthetic itself may be a little misleading in the fact that the fluid is still a Glycol based fluid as are all other DOT 3 & 4 brake fluids. But like I stated above, it is the other ingredients used and the specific formulation of those ingredients that makes one brake fluid better than another.
If you want to truly compare brake fluids to make sure you use the best available, you must look at the minimum boiling points of the fluids to determine which fluid is going to hold up better for longer so to speak. You will receive better braking performance from a fluid with a higher boiling point.
In the end, not all brake fluids are equal. Hope this helps a little...
---------- Post added at 11:33 AM ---------- Previous post was at 11:22 AM ----------
You can check out the Amsoil Dot 3 Brake Fluid MSDS to read more about the ingredients used in the formulation of the fluid. While not showing the exact formulation as that is proprietary, you can learn more about it for comparison to other brake fluids available.
Amsoil DOT 3 Brake Fluid MSDS