C&D becoming thinner over time only means that whatever factor makes it into a gel, for ease of application, has deteriorated. I had a bottle like that. It did not affect its efficacy at all. Just be careful how you apply it; don't allow it to run all over the place.
Some other very good cleaners, approved for use anytime, are Knockout, Zapp-It, and Vise Cleaner. New ones enter the market all the time.
Hook-it is more effective than any of those, and approaches the power of Clean & Dull but, like, C&D, it needs to be used as carefully as possible, protecting hands and skin by using gloves, just like C&D. It, C&D and LMB International cleaner are the only 3 strong enough to remove belt marks, but LMB is no longer being made.