You need to make sure the PH of the coolant is ok (PH>7) as there is a risk
that corrosion starts to damage the head and head gaskets.
The industry standard for checking electrolysis is with the engine running not to exceed .3volts. BMW, Mercedes and Porsches all have similar guidelines. Even with fresh coolants, it would have exceeded .1 volts for sure.
The test is simple: take a DVM, put the scale on millivolts, hookup the positive wire to chassis gnd, submerge the negative wire into the coolant reservoir and take reading with engine off and engine running.
Anything close to 0,3v (300mV) is a reason to replace the coolant right away.
Of course you can also use a PH meter or test strips.... PH>7 is what you are looking for.
1992 928GTS midnight blue