In case it helps anyone:
The owners manual for my late Gen3 says to drain the coolant from the plug on the bottom of the radiator when cold, empty and refill the expansion bottle to the full line and refill the rad to the top.
I've done that using Suzuki 50/50 longlife coolant (same as was already in the system) to avoid compatibility issues.
It took 4l so estimate there was nearly 2l that didn't get drained.
I've done the change at 6 years so what came out looked pretty good.
I think doing it like this before the coolant goes bad is a good way to go if you can - it couldn't be easier.
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