You don't need sealant. The biggest problem is keeping the seal in place for correct fitment. Most garages (RS-based knowledge-wise) recommend a bit of glue on the seal to the sump-side but only a small amount to hold it in place, then torque as per fitting instructions, working round it in the correct order. The glue is only to hold it in place so it sits correctly when offered up and doesn't replace the seal functionality.