I rarely use additives but Lubegard was recommended by a long time transmission shop. It slowly stopped a hard banging shift.

Fo years GM recommend Shudder Fix to stop torque converter shudder, it worked well.

What about additives for limited slip diffs sold by an specified by GM?

IMO some additives work, some don't. I respectfully disagree with additives should never be used in oils.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer