There is a milky oily residue under the radiator cap but not under the rocker cover cap or the dipstick itself. The residue is getting worse under the radiator cap and the coolant is even turning milky. I find it strange that the oil stays fine? What do you guys think? Oh and its an l20b and has 250 000 kms on the clock prob needs rebiuld soon. Still runs and idles perfectly too

Sounds like oil is getting into the radiator.

Head gasket is not blown yet but it is allowing oil to track to water gallery. Time to change it.

Sometimes it's possible to nip this in the bud by simply retensioning the cylinder head. I have found that this worked on a couple of occasions when it was done at the first sign of trouble.
In both instances the bolt tension seemed surprisingly low, so give it a go as the only cost to you will be a little time. Be sure to do it with the engine COLD. This was done to try to forestall gasket replacement to a time when it was convenient, & not at a time when it was necessary.
On both occasions, the gaskets were replaced about six months later.