OP never said there was a broken part. Thermal breakdown can diminish the viscosity the oil, especially at that mileage if the fluid was never changed the fluid has been broken down which effects the fluidity and the oils ability to lubricate not to mention the additive have most likely broken down, including the anti-foaming additives which will prevent proper operation of the transmission and is a good indication of the need for new fluid, without proper lubrication the components such as the gears will have difficulty catching also the absence of the anti-foaming additives can cause aeration forming bubbles in the oil which can cause serious damage to your transmission. I'm not just a salesman speaking out the seat of his pants my friend

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Do your car and move up to Amsoil.