I've used Zymol 's seal conditioner and liked the way it works. I use 303 for regular trim use but the seals can benefit from a bit more help.
here is the info on it:
"Soft seals, those around doors, hoods and trunk lids, present a number of problems. They are constantly being flexed and crushed while being subjected to hostile environments. Zymol Seal is derived from glycerin, a natural plant anti-oxidizing agent. Seal notably prolongs the useful life of rubber seals by releasing trapped solvents and restoring original moisture. Note: A properly treated seal will feel like a dog’s nose. Contains Glycerin, Avocado Oil, Glycerl Stearate (derived from Glycerin), Cetyl Esters, Cetyl Cocoamide (derived from coconut oil)."
It ain't cheap though so you may want to just try the straight glycerin route.