Personally, I would be tempted to flush and renew the coolant in the system.
If you have not done this before, or have any record of it - then its probably good preventative maintenance anyway, as it could be as old as the car.
Im not sure if G48 would be miscible with the chemicals in the G12/G12+/G12++ coolant that is already in there.
A coolant change is cheap and easy to do anyway, you can drain the existing stuff from the bottom of the radiator - the current G13 coolant is around £9 a 1litre bottle - you can get away with two litre's, the rest is water.