3m 847. Like a marine version of weatherstripping adhesive. Made for rubber sealing. I use it for o rings, shady gasket areas, seals, and tank sender gaskets.
3M Nitrile High Performance Rubber and Gasket Adhesive 847, Brown https://www.amazon.com/dp/B008F8WD04/ref=cm_sw_r_cp_api_i_RqN3EbQK2PG9F