Best slide out lube??

They may say not to lube the mechanism but I think thats just a marketing ploy.
If you do lube them you'll find the slide moves much faster and more quietly.
Personally I use a spray teflon lube and I do it often. If I drive in dusty conditions or if I just drive very far, I wash the rails down with lube before extending the slide.
Slide still works perfectly after almost 10 years.

I wire brush the rails/teeth.....then spray the rails/teeth will a dry graphite lube. Goes on wet then immediately drys. Will not attract/hold dirt etc.

Also use that on rear stabilizers (worm screw/pivot points) and on front landing gear legs.

For the slide out hydraulic arms I wipe them down with a clean dry cloth and then give them a good wipe with a cloth that has been sprayed with silicone

Many manufacturers recommend that rails etc. NOT be lubricated but merely kept clean. At LEAST do not use anything that will attract and HOLD dirt and dust (any kind of grease or oil) or you may get the opposite benefit as you intend. Flake dry graphite might be a good option but only when used on a clean system.
Good luck / Skip

What does the owners manual recommend?

Mine says do not lubricate the slide mechanism.

I don't know that I would consider any one product best, but I believe the consensus would be any dry lubricant, as opposed to anythng with oil or grease.