DOT 3 vs. DOT 4

After changing the oil in my 318ti I was going to top off the fluids. The
brake fluid reservoir says only use DOT 4 brake fluid. The supply store
only has DOT 3.
Anybody know the difference or if DOT 3 can be used?
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"grinder" wrote in news:yzJPe.2473$
I had always thought you could put dot 4 into a dot 3 system but not the other way around, however according to this:
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'DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of such a mix being a lowered boiling point. In an emergency, it'll do. Silicone fluid will not mix, but will float on top. From a lubricity standpoint, neither fluids are outstanding, though silicones will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. Since few of us ride at temperatures very much below freezing, let alone at 40 below zero, silicone's low temperature advantage won't be apparent. Neither fluids will reduce stopping distances.'
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In my old E28 I discovered that DOT3 caused the rubber to deteriorate. I switched to DOT4 and the problem was solved.
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Sorry about the earlier empty message - finger twitch.
"Bob" wrote
That's not quite a crock. Dot 3 is Castor Oil - brake fluid has always been an organic oil to *PREVENT* the rubber brake lines from breaking down. You probably had aged lines or contamination.
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If this is the case why does it disfigure (deteriorate) plastic grills and destroy paints? Is it the petroleum base in the plastic that it doesn't get along with?
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