I flushed my coolant as it was full of sludge, two weeks later it's getting oily again but not quite as bad so I've flushed out for one more time today just to see if that helps.

As the oil in the engine looks OK and not sludgy and there is not any white exhaust smoke , I am suspecting a bad oil cooler could be the root cause. Can anyone tell me how to best check the oil cooler is okay and not leaking oil into the coolant water? Is there a rubber seal that should be replaced etc?

Thanks