Rebuilding Windows that have shrunken gaskets..

So some of the windows on our LD have severely shrunken gaskets that I'm sure are the original OEM stuff from '85. The prior owners over the years have applied black caulking to combat leaking -- from afar you don't really notice this unless you're specifically looking for it.. However, it's pretty noticeable up close. What does it look like? Unfortuantely I do not have any photos of our rig's issues but a picture I found at VintageTrailerSupply (a site for Airstream owners) had a photo showing what the issue looks like on a AS trailer's windows and this pretty much sums it up for my LD as well -- unfortunately I'm not sure what the replacement gasket material looks like.. Anyone know?

I'm figuring that IF I'm going to remove the window anyway to replace the existing Butyl rubber caulk that adhere's & seals the frame to the chassis, I might as well fix the other issue with each of these windows since I'd rather do everything at once instead of the possibility of doing it twice..