High vacuum grease is the grease to use. Use a small amount in the groove to retain the seal. Also, put a small amount between your thumb and forefinger and coat the entire seal. If the seal has shrunk, stretch it slightly so it fits naturally in the groove. If it has stretched, place the seal in the groove all the way around with the excess hanging out. Put the two halfs together and while holding them together with slight pressure, push the remaining seal into the groove between the halfs.