Transmission fluids

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I wanted to change my transmission fluid but my mechanic told me that after 100k i should just leave it for life, is that ture? He told me its better off just too leave it alone then to change it. I now have 204k on my truck and i would like to change the trans oil but if it better off just to leave it as it is, then i would just leave it alone. Also what engine oil would be good to use moible1, royal purple? and the weight?

Any recommendation or comments would be great

Thanks,
Mike