Ummm. There are 12 pins in that loader. If it's squeaking, that's where to look. Hydraulic systems are well lubed and rarely squeak unless failure is imminent.
Use it for an hour or so. The rust will wear off and it will quit squeaking. If it doesn't, a squirt of oil on each of the 12 pins will cure it.
If the oil level is too low, the cylinders will not move smoothly and the pump will get noisy until it can pull in some more oil.
Measure the the post length, width, and height. Subtract 1/4" from each dimension, calculate the volume in cu-in, and divide by 231. The answer is the theoretical number of gallons in the post. Subtract 10% from that and the result is approximately how much oil it should have in it with the cylinders retracted.
Bleed air by exercising the cylinders. A minimum of 2 full stroke cycles, out and in, will purge most of the air. Any air that is left will get purged with a few minutes of use. I usually cycle my system 3 or 4 times after a cylinder or hose change before putting it to work, but I work can mine a lot harder with my heavier GT than you can work yours.