Q: is it safe to change Transmission Fluid. Hasn't been changed in a long time. Currently the Transmission is working fine; no slip

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Transmission is working fine. No slipping or noise. Car did not run for 3 years and fluid has not been changed for quite a while. A mechanic told me (without seeing the car) that it was unsafe to change the Transmission Fluid and it would cause immediate slipping due to new oil being thinner than old. Is this true? If I should NOT change the fluid, what should I do as preventative maintenance? (additives?)

My car has 213000 miles.
My car has an automatic transmission.

I agree with the other mechanic. If the fluid looks dirty then this means the transmission was not serviced for a long time and if you service it then the new fluid will wash out all the clutch material that is being held together by the old fluid. You would be taking a big chance that the transmission will start slipping within a week after you service it.

A 75-point safety inspection is a good idea after long-term storage, and a qualified technician from YourMechanic can perform this for you at your home or office.

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