I am guessing some type of mineral oil The picture is a tit that has started to rust but other wise is in good condition. I believe that vegetable will not harm rubber so I have given this tit a good shot of Walnut Oil that was past the expiration date for consumption.Click to view attachment

that is because the rubber material was not water proof and the spine was made from mild steel. Since the factory discontinued them we had them made but with waterproof factory look material and stainless spine.
No one cared that the vw produced 914 would be around 40 years later so did not use the best materials available. Too bad, there would be a lot more of the cars around if they had,. The 914 ended just one year before they started to galvanize the bodies. What if....

Stainless steel

I always hope some energetic soul with the proper skill level would make a 914 body, doors, engine lid, and trunk lids in carbon fiber. Put a vin from a chassis that is beyond saving on it, and paint it up. If it is done right, you would have a rust free 914 forever!!

wow wow wow

If I was where I am now back in 1978 when I started our autoatlanta business, I would have purchased every body in blank (white) the factory had and stored them in a warehouse. The factory offered 50 bodies to me back when Howard Adams was the parts guy for Porsche , but even at 5000 each, back then there were so many good ones still around that one could purchase for 3000, that I passed. They probably crushed them. (I know for a fact that they crushed many new factory front clips so would not put it past them to have crushed the bodies as well