The dealer advised that Ford is calling for Mercon V. Two dealers confirmed this. Tremec told me they are recommending Mobil 1 Multi-vehicle ATF. I had a supercharger like whine coming from the transmission that I knew wasn't right. I thought maybe I have a leak and my fluid level is low. I pulled the fill plug and bright cherry red fluid started ouring out so I am not low on fluid. It did not smell burnt at all. It looked brand new. I don't know what type of fluid it has since I got it 2000 miles ago. I filled it with Motorcraft brand Mercon V and I didn't realize how smooth it could shift. The high pitch supercharger like whine is gone. There is still some gear noise which I consider normal. There's a lot of conflicting/confusing information out there about what fluid to use in the WC T5. I plan on using Mercon V as long as it is available and as long as I have this car. I'm sure M1 ATF would work well also but I will stick to Mercon V since I know how well it works.