Tiling over old peel and stick adhesive.
Well that was easy.
I called MAPEI just now.
(Better than trying to figure out their website.
They recommend MAPEI ECO Prim Grip first.
Since I have Lowes, they said use Keraset/Keraply mix (Spelling?)
(Others are Kerabond/Keralastic which are available at tile stores.)
He did say ECO Prim Grip will be available soon at Lowe's.
Not sure when, but soon.
So if I want to do this ASAP, which I do, I need to call around for ECO Prim Grip first.
Thanks again everyone!