Hydraulic fuunction in earth moving equipment

1. The port reliefs are for over pressure protection to a specific actuator that can be influenced by external forces or intensification at elevated pressure that can damage them. If the port relief is set lower than the main relief then system pressure is also at that lower pressure when the actuator is operating. Anytime there are more than one relief valve in a circuit the lowest pressure will prevail.

2. This makes it impossible to mix fluids, put hydraulic oil in the engine, engine oil in the hydraulic circuit by some untrained person.

In my opinion it can also help cold weather start up and hot weather over heating problems when multi viscosity engine oil is used in the hyfraulic circuit.

Look at most hydraulic manufacturers oil specifications to see most have engine oil as one acceptable fluid.

Engine oil has detergents and other ingredients added to them to protect your engine. Hydraulic oil is usually between 10 and 15 weight. Not very good protection at high temperatures. You also don't want heavier weight oils slowing down the process of flow in actuators and valves. Smoother is better.