With having the flange problem behind me for now, I am getting close to 80K miles on my 2015 350 Tansit. This is a sealed system. I have been advised that you need to have the transmission "flushed" and refilled, or the trans will start slipping, and get torn up. How much of an issue is draining the trans, and putting fluid back in? I was also told not all dealers will do a "flush". Is this covered under an extended warrentee?