I had a customer that was leaving on a Circumnavigation and was concerned about the age (7+ years old at the time) of his perfectly performing Sonnechien Dry Fit German Gel batteries,  5- x 8D so a big bank.  He installed 5 x 8D Lifeline AGM’s and experienced numerous battery failures on the trip starting only 6 months out in the Pacific after departure requiring very expensive air shipments.  I am still using the 5 Sonneschien Dry Fit batteries removed from the boat, 0 failures so far.  We reinstalled the same upon his return to the US and had no further problems up until the time he sold the boat about 8 years later which puts my batteries over 20 years old.  Fort Lauderdale Battery used to sell the Sonneschien and still had some good specimens of this same type the last time that I talked to them which had lasted as long plus a lot more experience than I if you want to inquire. ( I have no connection with this firm other than battery purchases)   The dry fit batteries have been replaced by an Exide "Traction” Gel Battery which has the same case and supposed to be essentially the same battery.  No problems so far but my experience so far is limited to 3 years with these batteries as they are relatively new.   The Dry Fits were touted as being having very low gassing btw.  Despite some abuse, I have never seen any of these swell, crack or ever leak anything.