See-through viewing window for oil is not see-through anymore

[kriszzzz]

Hi, I have a bit of an issue as I can't see how much oil I have in the engine. The viewing window has just gone dark grey over the years, I can't see through it now. I have recently changed oil & filter and thought that the window should clear up with having the new oil in, but it is still dark. I put the recommended amount of oil in it, so the oil level should be OK, but obviously, I want to be sure that the level is ok.
Any advice?