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Brake Fluid Compatibility

I recently replenished my supply of DOT3 brake fluid stored in my garage, but noticed the new bottle was labeled “DOT3 Synthetic”, made by STP. I don’t know what is meant by “synthetic”. Will there be any compatibility problems if I put this in my car that already has a non-synthetic DOT3 fluid?

No, none. any DOT 3 can be mixed with DOT 4 or DOT 5.1 (glycol based) fluids but not DOT 5 (silicone based). BTW, all brake fluid is essentially “synthetic” or man made. Glycol doesn’t appear in nature.

The STP web site lists a DOT4 synthetic, but not a DOT3 synthetic. The major difference in the composition on the safety sheet (very complete) is the DOT4 had “Triethylene glycol monomethyl Borate Ester”, the DOT3 doesn’t.

Like @Mustangman said, they’re compatible.