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I am having some problems with rubbing disc brakes and came to the conclusion that it might be because of "sticky pistons". I would like to clean and lubricate them, as here.

This video instructs to use the same brand oil for cleaning them as is used in the hydraulic system. These brakes use mineral oil. Questions: 1. Is it safe to use another brand's mineral oil for this task? 2. Is it safe to use different mineral oil, e.g. watchmaker's oil?

This question is only about cleaning and lubricating the pistons, not about adding more oil to the system. I am aware that using DOT fluid is not safe—the question is about using other kinds of mineral oil.

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    Motivation (in case you wonder): I am reluctant to buy a full bottle of oil when I only need a few drops. I do not plan to experiment with brake bleeding in the near future, so I would not be able to make use of the full bottle. I am not able to borrow the correct brand oil either, as my friends have different brakes. I am also not sure that lubricating the pistons will resolve my problem, so I'd rather not buy a full bottle.
    – Szabolcs
    Oct 7 '20 at 13:11
  • I don't know the answer to the question as asked, though I have my suspicions that mineral oil is sufficiently inert for it to be fine. But it sounds to me like it'd be best to take the bike to a shop and let them handle it.
    – Paul H
    Oct 7 '20 at 14:58
  • Probably you don’t need to be too concerned with different oil brands for the minimal quantity you’ll be using to clean the pistons. This guy put baby oil in Shimano brakes for a year and couldn’t tell the difference. youtube.com/watch?v=63VIuPiX3CA Oct 7 '20 at 16:27
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    Hmm... makes me wonder what other oils can be mixed. I'd love it if my brakes could have a nice cinnamon smell after a long descent. :-)
    – Andrew
    Oct 7 '20 at 17:04
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    Bleeding the brakes mixes oils at the boundary, and leaves an layer of old oil behind in the lines and calipers so you have more than a trace of the old oil in the system. Piston seals wipe the surfaces leaving microscopic traces. If enough oil is transferring past the seals in either direction, you have some serious problems beyond mixed oils. Think what happens when you ride in the rain, does water get into the calipers? As long as the cleaning fluid does not damage the seals, anything will be just fine.
    – mattnz
    Oct 7 '20 at 23:57
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  1. Probably yes.
  2. I am expecting them to have very different viscosity and boiling point, so I would rather not.

The idea is to have an homogeneous oil if some droplets are left over after the cleaning process (same oil in same oil).

However, it should be no problem if you use a different mineral oil. At worst, the different oil will hamper your braking effectiveness, at which point you will need to flush out all the oil from the brake circuit.

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