My old AI Nikkors always had very light focusing, from new. It is a little disconcerting, but they were precise. I got used to it quickly, coming to appreciate the ability to one-finger the focus ring if need be. Slop is clearly due to wear, but considering the lousy feel and wobble in so many auto focus lenses, I figure a little looseness is not that serious.
One of my Pentax SMC-M lenses, bought used, had some wear, causing a slight wobble. I asked Eric Hendrickson (one of the gods of Pentax repair), and he said it would not be a problem, and wasn't worth the cost to fix. Just for the heck of it, I took off the helicoid part (it is a zoom, and is front group focusing, single helicoid). I wrapped some teflon tape around the inner part, stretching it tight to thin it. I put the helicoid together, and worked it back and forth a few times. Then I lubed it and reassembled it. The tape filled the small gap, and the lens still feels great several years later.
Thicker grease will also fill the gap, but I wanted to keep my focus feel the way it was, plus I didn't have to buy thicker lube for just that one lens.