The usual procedure for changing the oil in pneumatic spearguns is to let all the air out, unscrew the muzzle and then pull the piston out of the inner barrel. After that you can tip the gun upside down and pour the oil out of the inner barrel. Guns with a power regulator block tend to trap oil in the front tank, so to remove everything you can twist the nosecone off and then pour the oil out of the front tank at the same time as you empty out the inner barrel.
This alternative method uses the air pressure in the gun to blow all the oil out of the rear inlet valve and to do that without making a big mess you need a gadget like that shown in the diagram below. This device was developed for the "Taimen" which has a thick plastic valve stem, so you don't want to push that stem with anything sharp, instead use just a smooth ended rod that fits neatly in the inlet valve port. The gun is held with the muzzle pointed upwards and vertical and then pushed firmly onto the vertical rod which cracks open the rear inlet valve and the oil sitting in an accumulation in the rear of the gun is all blown out by the air pressure acting above it. In a big gush of air escaping all the oil is blown into a receptacle such as an old ice-cream container for your subsequent inspection to see if there is any water or foreign materials contaminating the oil that would necessitate gun dismantling to investigate the source of any problems.
If the oil looks good then you discard it, put fresh oil in a glass bottle or jar and suck it up with the hand pump and attach the gun to the pump, the gun still being held muzzle up and then invert the assembly and with a slow firm push press down on the pump handle and send the oil in the pump into the gun. Repeat as necessary to ensure the correct amount of oil is replaced in the gun and that is all there is to it. Make sure that the gun is set to full power when you do this as compressed air pressure can act on all the oil sitting in the rear end of the gun.
Note that a slow push is required as you don’t want the oil being forced in too rapidly which may blow the inlet valve “O” ring out of position, an unlikely outcome, but better to be safe than sorry.
Periodically the piston needs to come out to check for any wear and scratches on the piston and its seals, but this is unlikely unless you dive in very dirty and gritty water or spear in the shore break where schools of fish often run through feeding on material being churned up off the sandy bottom. I have bumped off quite large flathead doing this when I saw them zipping around my fins while standing on the bottom, so swam out and then turned back to open fire on the fish which are very hard to hit, so more ass than class in hitting one.