My real life experience on this topic, daily all year driver in swiss climate (cold winters/hot summers) :


An AGM battery (Bosch) had already been installed by the previous owner: I haven’t had any problems at all in the 1.5 years I’ve owned it, even though the car has a lot of electrical components and even an auxiliary heater that’s used frequently in the winter. The M57 diesel engine is also a bit more demanding when starting up.

So I’d say, yes, an AGM battery is compatible with our E53.