I'd never use Dot-5 in a car unless it was designed for it. Between potentisal rubber seal incompatibility, difficulty in bleeding air bubbles, potential water damage and the need to completely flush the system it doesn't seem worth it. What is the expected benefit? Regardless of the type of fluid it should be changed at least every year anyway....depending on driving style, tracking the car etc.
After that the brake lines (rubber) should be replaced periodically as they "stretch".
My recomendation is to use a pressure bleeder so flushing and/or bleeding on a regular basis is simplified and use a high-quility brake fluid. Like ATE Super Blue (if you can get it) or Castrol SRF (if you can afford it).
'85 380 SL (sold)
'85 Carrera Flatnose
'71 280 SL Signal Red/Cognac