Can the transmission of Nissan Xterra 2005 model be repaired if faulty?

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I bought a used Nissan Xterra 2005 which seemed to be in good conditions. A day after, I changed the engine oil and transmission oil. About 2 weeks later, I noticed the "service engine soon" light come on and then the car slows on pick-up and finds it difficult to climb even slightly hilly places. Reverse gear and selecting higher gears on speed all seem ok. Only starting off is the problem. I got a Nissan autotechnician to have a look and he said the valve body has problem. He said the problem is because I used the wrong transmission oil and has affected the transmission's valve body. He said the valve body can be repaired or I have to replave with another transmission. Before I ask him to repair it, I need your expert advice please. 1. What could have gone wrong with the transmission? 2. Is the valve body repairable? 3. How long can I expect the repaired valve body to last before it needs replacement? I thank you most sincerely for your help and feedback.