In discussions with various people here I have learned that some people, in fact most people have used different counts at different times. Some people use different counts for different games and are apparently able to switch back and forth rather easily.
That was not the case with myself. Two years ago, I decided to switch to a level 2, RPC after using hi-lo for about 4 years. I found making the switch extremely difficult. I play a good deal of blackjack per year, well over a thousand hours per year. Closer to 5000 hours of actual play over those 4 years, and you could probably double that counting all practice time. Hi-lo was so ingrained in me, that I am not ashamed to say that I had great difficulty re-training myself. I played RPC for more than a year and never did become as comfortable with it as with hi-lo. Oh I could keep the count, but it took more effort. I was not able to keep the count with a fast dealer, while watching the football game and have a conversation all at the same time as I did with hi-lo. And even a year later, sometimes when I wasn't concentration on what I was doing, I would find myself slipping back into hi-lo. So I made the decision to switch back. I know I give up a tiny bit of EV by playing hi-lo rather than a level two, but I am comfortable that I close that gap some or maybe even entirely by being able to play nearly error free and having the count be so second nature that it free's me up to take advantage of other opportunities that I couldn't before, and notice things that I otherwise wouldn't.