hi, I just got an old gpz250 and to check the oil I'm supposed to look through a little window. but the bike is so old the window is opaque and I can't see a thing. is there another way to work out how much oil is in there? in theory its just been serviced so I don't want to have it done again unless I have to. It doesn't seem to have a dip stick, I'm still trying to chase up a manual and I can't talk to mechanics until monday but I want to ride it tomorrow.