My dad is looking at putting dual batteries in his 80 series. He was doing some reading and found that some of the battery isolator systems say to not mix a gel type battery with a regular wet cell. He doesn't want to buy another Optima for a 2nd battery.
Optima's are AGM and have similar charge properties to a wet cell, mixing them would be fine. I would not mix a true gel cell with any other type of battery, they have very different charge characteristics.