hydrolics

ive just bought an early 70's massey 12 with a johnson front loader, ive never owned one and i am wondering how to add oil to it, it has a spot on tower that appears to be a fill spot with a wobbley vent, is the system under any pressure or can i just open that up to ck,

Reservoirs are not under pressure. Just the opposite as they must be able to breathe freely to the outside atmosphere. That's why there is a vent on that tower.

Sometimes,the vent is also the fill cap for the reservoir and sometimes there is a separate fill plug on the loader tower. Sometimes, both loader towers are used as the reservoir and there is either a hydraulic hose to connect them or the structural steel of the loader frame provides a path for the oil to balance out inside the two towers.

Most oil fill plugs are at least 1/2" in diameter and are often 3/4" to make it easy to put a funnel in. Look your loader over carefully. You likely have a separate pump driven off the engine PTO to provide power for the loader. Trace the hoses. One hose from the tower will go to the pump. One hose from the pump will go to the loader control valve and another hose from the loader control valve will go back to one of the towers. The other lines will go to the cylinders on the loader.

If you look carefully, you should be able to figure out if one tower or both towers are being used and then you should be able to find a drain plug as well as the fill plug.

What are you using for oil? You might be wise to drain the system and refill it.