I have had a little experience, not much of it successful. Generally, chemical cleaning does not do well, and lead is so soft that brushing must be done with very soft brushes and then gently.
Ancient lead is usually covered with white or whitish corrosion products. If these corrosion products are hard, apparently stable, and not obscuring significant details, there is no reason to clean them beyond gently brushing off adhering dirt. If the corrosion products are powdery, the lead may be brushed lightly with a soft bristle brush. Afterward, the seal should be soaked in distilled water changed daily for at least a week and then dried until a warm light.
If a seal or lead coin is deeply encrusted, it may be cleaned chemically or, in extreme cases, with electrolytic reduction, which can only be done practically in a laboratory. These techniques only occasionally result in greatly improved legibility. Here is a basic bibliography on cleaning lead: E. R Caley, "Coatings and Incrustations on Lead Objects from the Agora and the Method used for Their Removal. Studies in Conservation," Vol. 2, no.2(1955), pp.49-54; R.M. Organ, "The Consolidation of Fragile Metallic Objects," pp. 128-134. in G. Thomson, ed., Recent Advances in Conservation (London, 1963); H. J Plenderleith and A. E. A. Werner, The Conservation of Antiquities and Works of Art (Revised Edition, Oxford, 1971). For a good summary of techniques, http://nautarch.tamu.edu/crl/conserv...ual/File14.htm
After a lead coin or token has been cleaned, all traces of cleaning chemicals removed, and the seal properly dried, it is wise to give it several coatings of a microcrystaline wax created to preserve archaeological materials, such "Renaissance Wax." Lead is very susceptible to the corrosive effects of organic acids. Lead items should be stored in sealed containers and never in close association with oak cabinets or drawers.