Under the RFG regulations, RBOB cannot be mixed with RFG. However, normal tank blending of oxygenates will require the presence of residual "heels" of RFG from prior batches of RBOB and oxygenate blending, or routinely large portions of the preceding batc

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EPA would not treat the prohibition against mixing RFG and RBOB as having been violated in the situation you describe as long as the volume and oxygen content of the residual RFG from the prior batch have been accounted for by: 1) determining the volume of the portion of RFG left in the tank prior to blending with additional RBOB and oxygenate (the oxygen content of the residual amount will have been determined previously); 2) sampling and testing the entire tank subsequent to blending the additional RBOB and oxygenate to determine its volume and oxygen content; and 3) mathematically subtracting the volume and oxygen content of the residual from those of the entire tank to determine the new batch volume and oxygen content.(12/5/94)

This question and answer was posted at Consolidated List of Reformulated Gasoline and Anti-Dumping Questions and Answers: July 1, 1994 through November 10, 1997 (PDF)(333 pp, 18.17 MB, EPA420-R-03-009, July 2003, About PDF)