The 2013 uses the "blue" Subaru premium super coolant. It's supposed to last up to 11 years.
If this is for the occasional "top up", given the long time frame, why not pick up a jug of the Subaru coolant at your dealer? That way, there's no risk of mixing formulations, and not even a potential for impact (as limited as it might be) in case of warranty claims related to the cooling system.
The coolant comes pre-mixed (50/50).
(Perhaps stop by the service department of your dealer and speak to the Subaru mechanic. Many dealers are now using the blue coolant only, even for earlier model year cars that originally had the green fluid. When refilling cooling systems they usually end up opening a second gallon jug, but using only a small part of it -- the rest is kept in the shop and used for "top ups" (usually done during unrelated service, oil changes etc). You might be able to get one of the partial jugs, or they might fill a clean container (perhaps a 2 quart size), just for asking.)
incidentally, the blue Subaru super coolant jug I have says it contains "ethylene glycol" (in the "in case of emergency/medical warning section on the rear label).
It sounds to me that it would be a bad idea to mix Subaru Green and Subaru Blue, is that correct? I need to pick up a bottle for top-ups, too. Next time I have a cooling system service done, should I switch to Blue? Is there an increase risk of wear on the already fragile 2.5 liter head gaskets? (a la the GM Dexcool problems in the early 90s with intake gaskets?)
Subaru itself has indicated that the two coolants, blue and green can be mixed, however, the coolant has to be serviced (replaced) according to the shorter "green" schedule rather than the extended "blue" schedule.
From March 2008 TechTips:
The 2008 Impreza WRX STi and all 2009 MY vehicles are factory-filled with the new Subaru Super Coolant. This new coolant is blue in color and has an extended life of 10 years before servicing is required. It is OK to mix Subaru Super Coolant with Subaru Long Life Coolant, but if you do, use the original 30,000 mile replacement interval.
The new Subaru Super Coolant is required for all dealer warranty repairs to 2008 Impreza WRX STi and all 2009 MY vehicles that involve the draining and/or refilling of the engine cooling system. It should also be used when topping off of the cooling system, when installing an engine block heater, or refilling the cooling system after a collision that damaged the cooling system.
The part number for ordering purposes is SOA868V9270, which represents one gallon. This product is a 50/50 mix and does not require any mixing with water. This product must be ordered in cases of 6 gallons. For dealers in extremely cold areas that require extra protection (below -34 degrees F), Subaru Super Coolant Concentrate is available in one quart bottles and can be ordered using part number SOA868V9260. . . ..