SPCC applicability thresholds for aboveground and completely buried oil storage capacities are established in 40 CFR §112.1(d)(2).  If a facility has a tank that contains a mixture with oil as an additive, such as oil-based paint, should the entire mixture be counted towards a facility’s threshold capacity?

Yes; if a mixture contains oil as an additive, the entire mixture is generally considered an oil for SPCC purposes. The percentage of oil in the mixture is not determinative for the purpose of the definition of oil. When determining threshold calculations, a facility owner/operator must count the total storage capacity of the container, rather than the operational amount of oil stored within. It does not matter whether the container is actually filled to its capacity.