Oily substance in radiator

[B777-300ER]

Just pulled the cap off the radiator of my VY V6 to notice an accumulation of a brown sludgey substance on the radiator cap, inlet neck, and quantity test port. Car has not been overheated or anything, and runs well so i dont think its a H gasket. Also doesnt seem to be any water in the oil.
Ive chucked in some pics aswell

Any ideas?

[airstyle]

99% sure it's just a poorly maintained cooling system. Really old coolant will turn brown and eventually turn into a sludge - Give your system a really good flush(or 3) and replace that rusty radiator cap with a new OEM one ASAP. If you want to do it right take your car to a radiator shop to have your system power flushed and your radiator rodded too, if you don't do this you won't be getting rid of all the sludge in your system.

[Grennan]

Yeah it looks like the radiator has some green in it which kinda suggests to me that coolant is ####ing old and the radiator is getting really really dirty. Give the radiator a really thorough clean, flush it out a few times make sure to get all the hoses nice and clean.