I believe the original question was not about brake fluid types, but about contamination from petroleum products. Accidental contamination, one would presume...

I have read that the rubber parts will react badly to petroleum contamination, even when small amounts are involved (as you say). The dissolved petroleum is said to breakdown the seals. Not sure if it changes swelling characteristics, or literally decomposes the rubber, but it is a bad bad thing... Glycol brake fluid will dissolve oils. It is actually a pretty good solvent. Test it sometime! (Just keep it away from paint). It probably would not have a direct and immediate effect on your braking. The boiling point of oils is also very high.

If you accidentally poured oil in the reservoir, I would immediately get as much of it out as possible with a baster, then flush the system with fresh fluid multiple times. Being a good oil solvent, the fluid should work to flush the oil out quite well. Perhaps with no measurable seal damage... but no promises.


With great embarassment, I will confess:

I actually did the opposite once, and poured brake fluid into a hydraulic OIL reservoir. Really bad idea! (Hey, it was gray market, and all the labels were in German, and I was like 19 then, and what an idiot). Got as much out as possible without resorting to a total disassemble to drain, but every component in the hydraulic system failed within a few months. Brake booster, steering rack, hydraulic pump... Big and very costly mistake! Doh!

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1986 300E 5-Speed 240k mi.