My story is similar to yours. My 2000 ML had regular oil that was changed at 5K, Then got synthetic at around 10K. But, since then the FSS has counted down to 2K after about 7500 miles traveled. Knowing that the dealer would change the oil anytime after FSS reached 2K, I had it changed. I don't think your interval would make any significant difference. I too plan to drive the ML for quite a while longer. I've never had any oil consumption so I'm not concerned about the oil changes and longevity. The rest of the vehicle's parts are another story!
PS. you can do your own oil changes for about $50. But, you'll need to develop a home-made oil funnel or get a top-sider oil extraction tool to avoid a big mess.