2017 Subaru Outback: can I mix blue and green coolant?

Short answer: buy the blue and leave the green on the shelf.

Your Outback came from the factory with Subaru Super Coolant, a blue long-life formula (phosphate-OAT chemistry). The green "universal" coolant uses a different inhibitor package, and Subaru does not formulate the blue to be mixed with other types. Color alone never proves compatibility, but the label claim on universal jugs is about not causing immediate damage, not about matching Subaru's spec.

Mixing won't blow the engine the day you pour it in. The real cost is that you wipe out the long-life service interval and weaken the corrosion protection the blue formula was designed for, and mixed chemistries can react and form gel or sludge in the system. Once it's mixed, you have to treat the whole fill as conventional coolant and service it on the shorter schedule, so there's no real savings.

The cheap correct move: buy a blue coolant made for Asian vehicles (Subaru, Honda, Nissan spec). Same phosphate-OAT chemistry as the Subaru jug, half the price, and every parts store stocks it. Match blue to blue.

If you're stranded with a low bottle, a splash of distilled water gets you home safer than a splash of the wrong coolant. Top it back up with the blue afterward.

And one thing worth checking: at 86k miles you shouldn't be losing coolant. If the level keeps dropping, look for a leak. Slow seeps at hose clamps and radiator end tanks are the usual suspects.