US base oils supply fell to a four-month low in September, hit by a sharp slump in naphthenic production and a pullback in imports that tightened availability heading into the fourth quarter.

Total supply, or domestic output and imports combined, dropped to 6.06 million barrels (854,000 tonnes) in September, EIA data showed.

The fall marked a steep reversal from August’s four-year high above 7.20 million barrels, which triggered the largest build-up of surplus volumes in almost a year.

September’s decline partly unwound that build-up, limiting the size of any overhang heading into the fourth quarter of the year, when demand typically slows and prices come under downward pressure.

Maintenance work on a key Group II base oils plant at the start of the fourth quarter likely delayed for longer any significant build-up of surplus supplies.

US base oils prices remained unusually steady versus feedstock prices through most of 2025, including the fourth quarter, reinforcing signs of a more manageable supply-demand dynamic.

Such rangebound margins contrasted typical downward pressure on price differentials during the final months of the year.

A slump in naphthenic base oils output was the main driver behind September’s supply contraction.

Production fell to its lowest level in more than three decades, coinciding with planned maintenance on Ergon’s Vicksburg naphthenic unit, which began in early September.

Paraffinic output also softened, with production in the Texas Gulf coast district slipping to a three-month low.