It could well be your oil cooler leaking!
If there is a sort of toffee like deposit inside your expansion cap then this is what it is.
If it's just muddy looking water then it's just rusty and neads a good clear out.
If it is your oil cooler then this will need replaced, obviously an oil change should be done at the same time.
Either way your coolant system will need flushed out.
A good way to do this is if there is oily deposits in there is to drain the system, then refill with water mixed with a good degreasant
then run it up to temp, let it cool a bit then drain off. Repeat several times until clear. This will not damage the system.
Once you are happy then refill with antifreeze/coolant with the correct mixture of water.
Oh and
NEVER EVER THINK ABOUT USING RADWELD It's evil stuff
I blame losing one of my engines on that stuff. The head gasket blew and the piston over heated and seized the engine. And when I took the head off, the gasket had blown by some blocked cooling holes in the block, guess what had blocked the cooling holes? That's right it was Radweld.
So don't use it!!! It blocks up where you don't want it to. And you'll find you need to replace a radiator soon after anyway.
