Knife valve question

[Delphinus]

Ok this might just be a dumb question but I'm going to ask it anyhow.

I was thinking of using a knife valve (it's like a gate valve only it does up and down). I was looking at one and I could tell that the actual "knife" part (the plastic that goes up and down) is coated in some kind of lubricant.

Any idea what this lubricant might be? I'm thinking that this means the valve is not appropriate for use on an aquarium. The only times I've seen valves like this is on hot tubs, pools and RV toilet tanks, so I'm not sure. Its too bad because they are nice and compact otherwise.