7's would be better if your having the map
With regards to coolant types, just about every coolant is nap free these days, but I'd stick to an organic type as ethylene glycol coolants tend to eat water pump impellers after a while,
As for the flushing I just stick a hosepipe in the header tank and let it run for 10 mins with the bottom hose disconnected, never had any problems.
Subaru coolant is quite hard to come by these days my local dealer didn't have it when I enquired they just said analysis decent long life one would be OK
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