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1971 Mark III Transmission Fluid – To Change Or Not To Change

Bill –
The Transmission Fluid has never been changed in my Mark. Should this be done? Vehicle has approximately 45,000 miles on it.
Hello Bob –
You have a 1971 Mark III with only 45000 miles on it that sounds like a real nice collector car and a keeper. You haven’t stated if the transmission is leak free, shifts nicely or if the transmission fluid is clean and odor free ( not contaminated or burnt smelling ). I assume that at that mileage it still shifts perfectly and is good in all respects. If that is so and if it was my Mark, I would remove and clean the oil pan as well as drain the oil in the converter. All of the oil is then replaced with a fresh new fill of fluid. This is called ” good preventative maintenance”. If the transmission fluid in your car is burnt and/or the unit is not operating very well an overhaul may be in your future sooner or later anyway. You could still change the fluid to possibly improve performance or delay an overhaul. Many would advise that if the fluid at 45 years old was never changed you will only cause the transmission to fail faster if you change it at this time. I do not personally believe in that rational at all. If you own a real nice and worthy collector vehicle that you are proud of and want to maintain it in optimum driving condition the oil should be changed. If the transmission is in such bad shape that a fresh oil change will only quicken a failure I would like to know that on my vehicle and expose such a problem. If the transmission would then need an overhaul then I would have it done. This is all part of the old car hobby. You could also consider visiting a few good and trusted local transmission shops and have them assess the situation up close for an on scene opinion.
All of the above is of course only my own personal opinion from experience with several C6 transmissions that I have owned and serviced for many years. If you visit the internet you can find many forums regarding this matter with hundreds of other opinions on that subject as well on types of oil to use. To sum up it seems to me that only cars with transmissions that have been abused and/or that are already needing an overhaul because of age and mileage are the ones that are affected after fresh fluid is introduced into their system. We wish you good luck with your 71 Mark.

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