I have a 04 WRX with about 17.5k miles. I checked the engine coolant reservoir and it shows that the coolant is a little below the low mark. I'm confused as to if i can directly fill the reservoir until it hits the full mark or if i am supposed to fill the reserve tank on the left side of the manifold. Also, I've read posts where distilled water will work fine but im confused if I should use distilled water or if i have to use OEM Subaru Coolant with the Subaru Cooling System Conditioner.

top it off so it's in the middle, it's normal to lose a bit since the water in the coolant will evaporate

buy pre-mixed coolant at the dealer if you are concerned

Add to the resevoir as it's just an overflow and pouring into the main tain will probably create air bubbles.