There are five main classes of nylon: 6/6, Nylon 6, Nylon 6/12, Nylon 11, and Nylon 12. My guess is Nylon 11 extra flex extruded tubing because it has high chemical resistance properties and a long molecular chain when extruded for strength and flexibility.
Also Nylon 11 is also much less rigid than the type 6s of the resin. It allows for a lighter weight tube wall, greater flexibility and smaller bend radius.
There is a reason nylon is used for high strength fishing line, ropes, brush fibers for street cleaners, to tooth brushes, molded trigger springs for power tools and flexible tubing because these items are resistant to breakage after flex testing of millions of cycles.
The tubing also maybe polyethylene as it is the base material used in the red gas jugs you buy for your lawn mower and in the modern cars that have been using plastic gas tanks in place of the traditional steel. Polyethylene is also used in rope, the PEX hose for use in homes that will not break when full of water and frozen or when used with maximum temp for water heaters and heaters of homes via tubing in floors.
Another formulation of PE is used in the trash bag industry where the higher quality formulations are difficult to break even after stretching 10x1.
There are so many formulas of these base plastics that I can only guess which is the one being used
None of the quality formulations for tubing require strain relief fittings. Plastics is often miss used and therefore must be correctly applied. The materials coming from the Far East are some times of poor quality and poorly identified as to application.
If you are interested in knowing the actually plastic being used ask the suppliers: Mikes, Bratton’s, Snyder’s, Mac's and others what plastic tubing they are selling for this application.
Or buy the copper line that is rugged yet subject to stress hardening if not properly supported to dampen vibration.
The model A oil pump has a high volume but as the oil heats up the pressure drops to very little depending on where measured. If measured at the discharge close to the pump the pressure may be only 5 PSI when hot but only 1-2 PSI at the gauge mounted under the speedometer. When located on the top of the steering column the pressure may be zero.
Fact; with the flood oiling system the gauge PSI will diminish due to the length of the tubing, and the height of the gauge above the engine.