Another issue with AGMs is that they can't be topped up with water. So while they need a high charge voltage, charging current needs to be carefully limited to avoid boiling. If you boil off the water from an AGM then it's basically toast.

That's why I have a bank of AGM batteries attached to my solar system in the house, with an expensive multi-stage charger, and don't recommend them for an automotive application. Especially with an older car with a basic regulator.

__________________
1993 300D 2.5L Turbo
1991 300E, looking for diesel to swap in