For slipping automatics, I hear testimonials from customers--try Lucas ATF/Fix. Bottle is 24OZ. Change fluid & filter--add (1) bottle of ATF/FIX & the remainder of ATF fluid. Then drive for 30 minutes at regular operating conditions. I'm not one to believe in cures in a bottle; however, I have customer's (at my normal vocation), that say "this works". Lucas products are normally sold @ local truck parts distributors or NAPA. Also, I think Lucas has a website too.

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1999 Teal Blue F-150 s/c
4.6L, XLT, 3.55 L/S
Tow pkg., rhino liner, engine block heater & hard toneau cover. single c.d.
255/75 r16 Generals-owl