Why can't you see through it? it's prob filled up above the glass with black oil an when you drain it you will see in it again. the blaster don't even take a court of oil.
i just changed mine and he's right its about 3/4 quart just hope you have better luck than me i couldnt see through mine but that was because it had about half a quart of water mixed with my oil(vent plug came out )
i run a full quart of oil in mine to keep the push rod in the clutch lubricated. i was told it is always better to overfill a blaster rather than have to little.