which oil is better for car

Synthetic oil will handle temp extremes better than petroleum based oils. Have heard that on some engines that changing to synthetic oils can start an oil leak where there was none before but I feel that is likely not the case and that an oil leak may have been there before and unnoticed till after change to syn oil and the driver starts to pay more attention after changing to something new. But could be wrong. I am sure there are many opinions about that. Syn oil is more expensive but can go longer between changes. Do the math and see if pencils out. On a 1991 vehicle if has original engine with no major recent work on it just how much benefit is there?