If the fluid has been changed to silicone (DOT5) it should have a label on the reservoir saying so, in which case you would use that, but as your question implies there is no label it should be mineral fluid. I would buy some DOT4, and then compare the smell of that to what's in the reservoir just to make sure before adding it.
Standard brake fluid is not mineral oil based, DOT 4 and 5.1 are based on polyglycol ether based fluids. Mineral oils must not be mixed with these fluids as the seals in the braking system are not compatible With mineral oils. Only a few vehicles use mineral base fluids for the braking system, such as the Citroën with their hydropneumatic suspension that use a common fluid LHM.
If you have DOT 4 in the system already any fluid that meets the DOT 4 or later DOT 5.1 standard will be compatible, but not DOT 5 which is a silicone based fluid, unless you intend replacing all the fluid in the system.