[Closed Case]

Our 2015, Premium and Limited were both changed out around the 5th year. There were points when we had to add 1/4 gallon over hot summers due to temps, not each season just over all. Read up on how-to filtre out bubbles in the system. Look for cold shut down points and do not panic when the reserve point dips really low. Check it when fully warm. Overfilling is worse than a touch low then max. You can buy mixed OEM. But if you think it need water, make certain it is distilled.

The mixture is important to the head gasket integrity and can compromise that as well. Have the dealership check the mix level or a certified centre. Auto Zone has the ability as well as Monroe, NAPA, etc.

 

[Closed Case]

All my Subarus have needed a little added coolant once in a while... but I never use water of any kind. I use only the 50/50 premixed Subaru Super Coolant as specified in the OM. The top off to the Full mark is also done when the engine is cool.

That was 6.5 oz x 5 years. Daughters drove Cali to Connecticut R/T. Ourselves we used the mix on the Limited, they did not. Girls' Premium over 145,000 [miles] presently and no issues for their 5-speed manual. Dealer said, "..if you're a little low and worried, distilled..."

Sadly, many peeps are not around the corner from a Subaru Dealership crossing the US or Cali's Great Basin. We assume the 2015 Premium will get upwards of +/(-)250K before thinking sale/trade-in. It's Orange. That's great faith. Nothing lost engine oil, broke, or burned or baked like a Taco truck roadside, meh ....smelt the jalapeño like the mornings of napalm.

It's all good.