White grease is petroleum based. I use a product called "Syl-glide"...availailable at Peb Boys and (I think Auto Zone). This is a silicone grease intended specifically for maintaining/rejuvenating rubber materials, among other uses. Usually hanging near products in tubes (silicone gasket, glues, adhesives, etc.) Comes in a red, white and blue plastic tube.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)