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This weekend I'm planning to change the ATF (Automatic Transmission Fluid) of my 300E '86 that has 107KMi. I only had this car for about 6 months, and I don't have much info on past service. The ATF is a bit brownish, but is clean, no particles, and does smell normal.

Looking for some advise (this is my first Mercedes car), today I spoke with a friend that is a mechanic that usually works on Rolls Royces and Jaguars, but occasionally also works on Mercedes and BMW, and sometimes even on Chevys and Fords. I told him about my intentions, and he explained that since my transmission has over 100KMi he would strongly recommend that once the filter was changed, not to replace the ATF, but instead reuse it by filtering it before putting it back in. Yet another of the mechanics at the same shop independently also told me the same thing. They both explained to me that since the oil that is currently inside my transmission is "thicker" than fresh one, it is in fact making up for the increased tolerances due to normal wear of the parts inside the transmission. He insists that if I were to refill the transmission with fresh ATF, it would very soon start to slip.

About an hour ago I also spoke to another friend that is a very experienced DIY on Mercedes and other cars, and he suggested that I refill my transmission with 4 liters of fresh ATF, and for the remaining 2 liters (I understand the total is 6 liters) I should filter some of the old ATF and put it back in to up it's viscosity, and so make up for any wear in the transmission. When I mentioned what the other mechanics had told me about totally recycling the old ATF, he said that is indeed a old mechanic's trick but it is mostly done on other brands of cars, mainly American, but in case of Mercedes cars you can safely replace the whole ATF with fresh one, but... then he also added that if I wanted to play it safe, I should mix in some of the old ATF.

Any thoughts on this matter will be greatly appreciated.

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