We went round and round on the BMW Airhead list on this point. It's all AGM, not gel now. I had good luck with Panasonic AGM-same as what Westco sell relabeled. In the USA a company called Digikey sells Panasonic real cheaply. You can do a search for the 800 number and give them a call on sizes. Just put 50K tough miles on one. It is still very strong.
Bill in Tomahawk, WI