It's likely made of EPDM, or possibly a natural rubber blend. EPDM has become popular for belts and coolant hoses in the past couple decades. Suspension parts, too. It resists heat, ozone, and weathering better than many other rubbers, which is why it's so often used. Serpentine belts and timing belts used to be made from neoprene, but in the 90's manufacturers started converting them to EPDM to better tolerate higher underhood temps. The EPDM belts last longer, and don't harden, crack, and split like the neoprene belts used to. However, it has very poor resistance to petroleum/hydrocarbon oils. I don't know if the liquid mounts are made of EPDM, or of a cheaper rubber like a natural rubber (latex) and SBR blend similar to what tires are made from. Regardless, if you want to keep using the liquid mounts your only realistic option is to fix your oil leaks. Which really isn't a bad idea anyway.`
Designed them for max stiffness and minimum weight. Juggling the shapes and material thickness until I got about 1 mm of deflection under 1 ton vertical load each. The reason for this was to stop unwanted and uncontrolled jiggling up and down. Trying to keep all the movement in the isolators.
They went all to plan. But the plan had a flaw. I thought the mount bosses on the block were equal offset left/right (because why wouldn't they be right?). But they weren't so the engine sits more on one than the other.
I'll remake them next time I have the engine out.
Durabak makes naval deck coatings among other coatings, their UV resistant coating can be used to coat rubber and is chem resistant too so perhaps this will protect those hydro mounts against oil damage too....
Here are mine. I made them out of 3/16" plate I think. Actually, the plate piece is 1/2" that I got from eBay. Then I added the 3/16" plate for the 45 degree pieces. Minty welds tho! They are holding up just fine. IMG_20210422_155520 by Paul Abbott, on Flickr