I am going to drain and refill the automatic transmission fluid on my parents' 2000 Mercury Villager. It has about 93,000 miles on it and I don't know if the fluid has ever been changed. If there is a filter I will replace that as well. I am planning on refilling it with a synthetic fluid (maybe Mobil 1 ATF?) Obviously, draining it will not get everything out, so is it okay to mix the new synthetic fluid with the (probably) conventional fluid that's left in the transmission? Also, should I do this with the transmission hot or cold and does anyone know approx how much I should add? I have a Haynes manual but I don't see where it says how much to add for a drain/refill.