What are your thoughts on changing the Automatic Transmission Fluid on a 33 year old car that has the original Fluid and 50,000 miles.Fluid is Red and appears clean.I have heard a suggestion of draining out and replacing a litre of fluid every few thousand miles,instead of replacing the entire amount at one change,what is the reasoning behind this idea? is there any merit to this,or is it nonsense? Would you just replace all the Fluid,and fit a new strainer/filter in a single drain and refill,and maintain from there?