Car ran 40 miles with oil cap off. Pulled off road when oil warning light came on. Engine was steam cleaned & oil replaced. Lower fuel performance now. What is affected?
Possibly the air filter is clogged with oil - the oil that came out the filler may have been drawn into the intake, so inspect the filter and clean or change as necessary. If oil got past the filter you may also need to clean any sensors between the filter and engine, as these will affect fuel control and, consequently, consumption.