I would use genuine Subaru coolant from the dealership, 2 options as follows:
1. 1 gallon long life coolant + 1 gallon distilled water. Total cost around $25.
2. 2 gallons super blue coolant. Total cost around $40.
I used the long life coolant because the intervals (every 2.5 years or 30k) don't change whether you use either one on our cars and it's cheaper. The 2008 sti and 09+ cars benefit from the super blue coolant which intervals for the first 11 years or 137k and 6 years or 75k there on after.