A thrust washer is a bad thing to lost. When i overhauled my transmission the 4th gear washer was in 4 pieces floating around in the case. If you lost a washer, that means that hard parts are now running on eachother. I don't know if it means your clutches will start to slip, but it'* definately gonna put a lot of heat and stress on any of the hard parts that are now rubbing against eachother. <--- Bad. Keep an eye on your fluid and change it in another 5k miles or so just to see if there'* an unusually high amount of metallic flakes and/or chunks in the pan.
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