Replacing single reservoir M/C with dual reservoir on four wheel drum system

DOT 5 is not overall better, it's just different. Regular brake fluid (DOT 3, DOT 4, and DOT 5.1, in order of increasing boil point) is hygroscopic for a good reason: that way, a fair amount of the water that inevitably enters the system can be chemically sequestered harmlessly, slowing down corrosion in the hydraulic system to a pace slow enough that a periodic brake fluid flush, every couple years or so, effectively eliminates the problem.

DOT 5 is not hygroscopic, so the water which (again, inevitably) enters the system will coalesce into slugs at the lowest points in the system, where it will more aggressively corrode whatever it's in contact with. And if enough water coalesces near enough to the brakes themselves, like in the wheel cylinders, then that water can boil with brake heat, turn into steam, and oops, now you've got compressible gas instead of uncompressable liquid in the hydraulic system, and you don't got a brake pedal no more.

Use the (general) kind of brake fluid the system was designed and specced for. There's enough variety in what's available that you can surely find one that'll give you a warm fuzzy to use.