I've been using tire stuff for over 20 years and never noticed any ill effects on my tires but I'll definetly avoid the WD40.
What do you mean by the the outer most tread doesn't look good? I prefer control of applying with the Meguire's gel I use.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch