An experiment to test a new gasoline additive, Gasplus, was performed on three different cars: a sports car, a minivan, and a hybrid. Each car was tested with both Gasplus and regular gas on 10 different occasions and their gas mileage was recorded. Here are the partial box plots:

A two-way ANOVA with interaction model was run, and the following ANOVA table resulted:

A plot of the residuals vs. predicted values showed:

What conclusions about the additive and car types do you draw? Do you see any potential problems with the analysis?