[Jim/GA]

Shell labelled the Rotella T oils as suitable for both diesel and gasoline engines for many years, using the API star system. The EPA made them stop marketing the oil to gasoline engines because the phosphorus in the ZDDP additive poisons the catalytic converters used on those cars. (It was either that or significantly reduce the ZDDP levels, which older diesel engines really need -- so they changed the marketing materials.)

Shell did not actually change the oil, they removed the API S_ star for spark ignition (SI) engines on higher-ZDDP oils. So if your Model A has a catalytic converter on it, you should NOT use the Rotella T oil in it or the EPA will be upset with you. Otherwise I consider the Rotella T4 (conventional oil) or T6 (fully synthetic) 15W40 to be the perfect Model A oil for year-round use. Shell can't "recommend" it because the EPA has told them not to.