They were previously filled with Shimano mineral oil. Now they are to be re-purposed for dot 5.1 oil. Any issues with doing so?
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Technically you could, but it's a very bad idea.
The two fluids are incompatible and will contaminate each other, forcing you to re-bleed sooner than expected.
Whichever solvent you use to clean out the hose of mineral oil, will also be a contaminant for your dot fluid. In addition, you'll need some serious dedication and resolve to make sure you actually clean it out completely.
Labour + more dot fluid > Cost of length of hose.