This jug of conventional green automotive coolant concentrate was opened years ago and then tightly closed. Now it has a slight "oil slick" appearance (note the sideways V pattern in the close-up) with quite a few small distinct transparent globules floating on the surface. I can't imagine that this was contaminated with oil after it was opened, yet it looks that way now. Could this just be the result of sitting unused for years? Can I still use this?