I just purchased the CX-601, which is quite similar but larger in size. The oil window is present on my machine as well, but there is no oil in the head. The gear drive is greased with a medium/heavy grease. I suspect the same casting is used on a lathe or other machine which is oil filled, and so the hole is there and they simply fill it with a window regardless. If you want to confirm on your machine you'll need to pull the fine Z adjustment knob off, then the front cover. Behind the front cover you'll find a black cover with two phillips screws, removing this will allow you to see into the lower gearbox.
-Dave