Best brake fluid is one that suits your needs.
Some 5.1 spec fluids used to have a lower boiling point than 4, butt generally have a higher boiling point.
Downside of upgrading to higher spec, used to being more hygroscopic ( absorbs water ), this in turn requires more often fluid changes.
If you boiling current fluid using brakes ( if this is the issue? ), changing fluid unless it’s way old is just one thing to consider, other changes could be change braking use / increase cooling / increase size, also if issue is heat -worth checking calipers seals for heat damage ( are dust seals brittle / heat damaged, if they are – other more important seals could also be).
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