Yes in fact using the wrong coolant can cause problems major problems due to the mixing of different coolants and different chemical makeups in coolants $500.00 sounds outrageous for an external coolant leak. The car will need a coolant flush Id reccomend letting the mechanic do this seeing as two different coolants were mixed. But the price should be no where near $500.00 for the repair and flush look for mid 3 hundreds on the most expensive end. Take your car else where!!! Alot of mechanics dislike the Lumina therefore will bid high on jobs!!!!!!