Found this at Ask A Scientist.
http://www.newton.dep.anl.gov/askasci/chem03/chem03003.htm

Question -   Is glycerine a good rubber protectant?
Vince Calder & ProfHoff didn't have much good to say about glycerin as a
protectant.

YMMV

Steve

On 25-Oct-09 16:11 Roger VanTill sent the following:
> Hmmmm.....the Zymol Seal contains mostly glycerin.
> Nice tip, John.
>
>
> On Sun, Oct 25, 2009 at 11:46 AM, John Meeks <vanagon@gmail.com> wrote:
>
>> On Sun, Oct 25, 2009 at 8:45 AM, Michael Sullivan <sandwichhead@gmail.com
>>> wrote:
>>> Good morning, Volks.  Looking for a recommendation on what to use to
>> revive
>>> the seals around dors and windows.  They are in good shape.  Something at
>>> Napa or a home remedy, please.  Gonna wax today.  Thanks.
>>> Michael in San Antonio
>>> 91GL AT 'Gringo'
>>> 73 Beetle
>>>
>> Michael,
>>
>> I use Glycerin thinned with water. You can get a bottle at the drug store
>> for a buck or 3.  Rubber soaks it up. Keeps it pliable. It's not gonna give
>> you that ArmorAll car show look, but it does help to seal in the natural
>> oils.
>>
>> John Meeks
>> '91 Multivan
>> Northern Michigan
>>
>> Vanagon Rescue Squad
>> http://www.vanagonauts.com/Vanagon_Rescue_Squad74.htm
>>
>> Radio Call:KC8ZFN
>>
>