I am a fan of Delco batteries, but often forget that some over-the-GM-parts-counter replacements may require periodic maintenance.
The other day, I popped the caps and found them to be slightly low on water. Not suprisingly, the passenger/firewall battery was a little lower than the driver/headlight side, most likely due to extra heat soak it receives.
Added ~1pt of distilled water across both batteries. They have been in service about 18 months.