Oil & Fuel Iseki TL2501 oil sump leaking full of diesel fuel

[Isekitl2501]

First it was a small amount of overflow from the ventury tube (U shape) on top of the injector pump,
after not having used the tractor for several month, I started the engine and fuel/oil mixed oozes
from the oil filler cap. I do not know where to start finding the problem. The engine ran normal all the same,
and started ok. All I have is a user manual not much help. Your help appreciated. Keith in Oz.