Paraffinic base oils accounted for 86 percent of the first-half volume, or 27.3 million barrels, while the remaining 4.5 million barrels were naphthenic base stocks.

The heated pace of production also matched that reported for the second half of 2014, which reached 31.8 million barrels, too. One key reason for the year-long spike in supply, sources agree, was the substantial bump in capacity that arrived last summer with the startup of Chevrons base oil plant in Pascagoula, Miss. That added 25,000 b/d of API Group II capacity, and bulked up the nations base oil processing muscle by 10 percent.

Looking more closely at the two product streams, reported paraffinic volumes rose 12.7 percent in this years first half, versus their output in the first half of 2014. Paraffinic production was 27.3 million barrels in the first half of 2015; 26.8 million barrels in second-half 2014; and 24.2 million barrels in first-half 2014.

Naphthenic refiners did not enjoy the same growth spurt though, and actually saw some volume shrinkage. Pale oil output was slightly under 4.5 million barrels in 2015s first six months, versus 4.6 million in 2014s first half.

Its fall from last years second half was even steeper: From July to December, naphthenic production was 5 million barrels, or around 12 percent greater than in this years first six months.

The full repercussion of another new slug of capacity, ExxonMobils expansion at its refinery in Baytown, Texas, cannot be discerned yet from the EIA data. Baytowns expansion (which LubesnGreases estimates to have added 7,000 b/d of Group II) came onstream in May, and gradually ramped up production.

Data being gathered now eventually should give a clearer picture of Baytowns impact on U.S. base oil volumes. The EIA collects data on refinery net production of petroleum products, and aggregates it in the Petroleum Supply Monthly after a lag of two months.

EIA also does not collect production data from rerefineries, although these plants have roughly 5 percent of U.S. base oil capacity.