My oldest cube (25th anniversary) is superr loose and smooth, and I just found it yesterday (it was hidden away in the drawer and I never noticed that it was in the DIY box). After all, it went through a lot of stuff to get to this point:
In the beginning, I think it was already pretty okay.
-Then I decided to lube it with vegetable oil, because I couldn't convince my parents to go buy a lube.
-Well after a week or two I finally got the lube (Motomaster), and it didn't seem to make much difference though.
-I tried doing the center-stretching by using lollipop (from Halloween candies lol) to stretch out the centers.
-I got another lube (3 in 1 Professional silicon spray), and it somewhat felt a bit better...somewhat. After around August 2006 I bought a DIY cube, and the store cube was put into that DIY box, and into a drawer, which I never figured out until Feb 2008.
-Well, when I first turned it, it still feels pretty loose and smooth, but once I lubed it with my current lube, Jig-A-Loo, it was AMAZINGLY loose and frictionless, plus soft. Though of course, it locks up a bit.
So, I'm thinking of may be playing around, mixing cubies. May be that loose center can use new cubies and those nice worn out cubies can use a tighter center? Hmm