Originally Posted by
cyccommute
Lithium grease takes this "soap" or surfactant and uses it to dissolve up oil in the solid to make grease. The mixture is likely 90% oil and the rest the surfactant and/or other stuff. The resultant mixture will remain stable for a very long time. Over time and exposure to the elements, however, the oil can start to bleed out. The surfactant is left behind. None lithium grease can be a mixture of surfactants or it can contain clay or both. When people open up old bearings and find "peanut butter" or "grape jelly" inside, what they are seeing is the binder (soap or clay) left behind. The oil didn't "harden", it flowed away over time.
How long it takes for the oil to separate from the surfactant depends on how well the grease was emulsified in production. Cheap greases will separate faster than better greases. My experience is that good quality grease like Pennzoil #705 and Campagnolo grease can go years without separating, while bargain auto parts discount store grease can start to separate in a few months.
N.B. this separation issue is a reason why many old-school cup-and-cone hubs have an oil port where you can add more oil to replace that which leaked out. The act of riding will re-emulsify the oil with the surfactant and restore the grease.