Neoprene!
The automotive industry uses items such as brake cups, CVS boots, hoses and hose covers, belts, and ball joint and steering linkage seals that are made with neoprene. ( I believe they also use it for gaskets for race cars).
I bought my return hose at Napa they did not have the 3/8 hose(supply line) at the time.
The 5/16 return hose is the longset. Check to see where they attach to the metal line going to the tank. Some attach at bell housing some attach at frame.