Dish soap works well; however, some dish soaps might contain compounds/aromatics that might not be good for rubber-seals.
Nonetheless, Steele Rubber Products recommends "everyday dish soap and water".

Walmart has a Personal Water Based Lubricant Jelly in small tubes for about $3.00.
It has no additives, is fragrance-free, and is water soluble. It will not hurt rubber-seals.