car: 1996 GL / auto. / 2L / 97,000

problems: o/d light flash (highway only) and bang shift

I understand this problem...with great help from the members of this forum. I will do the 3x drain and fill and probably change the TSS. But nothing major.

I want to use an ATF additive. Any thought on this?
Which brand name and how much too use? I read that a product called LubeGuard is pretty good. And what about Lucas?

Thanks