Can you mix synthetic transmission fluid with regular transmission fluid?
Hands-down, you will be voiding the warranty on your transmission if you mix synthetic transmission fluid with regular transmission fluid. For example, a reputable car brand regularly notes in its owner’s manual, that if you must replace transmission fluid, you must use a particular transmission fluid that is a synthetic blend by their own company. The key here is that this type of transmission fluid will have the proper lubricants and protectants that are key to the proper functioning of the transmission systems, and some of those are very much proprietary items. They also warn you will violate the warranty if the proper fluid isn’t used.