1) Diesel are composed of hydrocarbon chains, CaCO3 is added as a detergent in oils, it collects polar groups and makes a salt. Thus; removes polar groups. For case of diesel they can react or trap if there is any polar groups in diesel
2) Diesel and oleic acid are almost same composition, oleic acid has polar group in it. They wont react with each other. Oleic acids will form a micelle inside diesel
3) As I said in part 1) oleic acid has polar groups and they generally attach to CaCO3 and form micelle. But these reactions happen at high temperatures like 150 C or above
I know these because I work with automotive lubricating oils. Read this book: