Fixing the rate of fire sears can be a bit of an adventure. There's a few tricks.
There was a video that shipped with these (or was available, can't remember which) that detailed all the maintenance and adjustments. Luckily we have youtube
For the fluid, any hydraulic fluid should do it. I picked up a quart (or liter) at my local farm supply store, though auto parts stores should have it as well.
It's been a while since I had to refill one of these so I'm going by memory. You may have to experiment a bit. This should get you close.
For refilling you want to make sure that no air is in the cylinders. Easiest way is to fill a bowl with the hydraulic fluid and put the entire rate of fire assembly in it for assembly.
Pull both cylinders off the assembly and turn the left hand adjustment screw (when facing from the front), which adjusts the rate of fire, most of the way in (but not all the way in), and the right hand adjustment screw, which I like to call the timing screw, in about half way.
Put the assembly in the bowl and make sure all the air comes out of the assembly. Insert one cylinder about three quarters of the way in. Now insert the second one about half way. The first one should start to come out as the second one is inserted. Take out of the bowl and wipe off.
Re-install the assembly and tune as per the video. If the sears don't engage turn the timing screw in. If they lock and don't release turn the timing screw out. With the rate of fire screw most of the way in the full auto rate of fire should be very, very low. You'll be able to see what the sears are doing, so you'll better understand how the whole mess works. Once you got that it's easier to figure out adjustments.