Another vote for Einzett Gummi Pfledge, it works great.
The absolute best weatherstrip treatment is BMW Gummi Pfledge but they discontinued it a few years ago, probably because it didnt meet the new VOC standards or somethng. If you can still find these tubes, buy them.
If not, the einzett product Barry mentioned is a great product too.
I know about dielectric grease, have used it in electronic applications where it works great and some automotive applications where it works great, but dont know what its made of. If it has alot of things that attack rubber, I wouldnt use it ever on moldings. I am curious to know why GM recommends it..
Good luck,
Dan F