I have exactly the same oil leak issue with a 12J engine in my 1985 Land Rover 90. It was fitted with a replacement 12J engine (from an ex MOD vehicle) about 10 years ago.
I have persistent oil leaks and have concluded that the crankcase pressure is too high and my focus is on the breather unit. Looking at your thread and the associated photographs, I have concluded this evening that the breather may be fitted incorrectly. In my case it is mounted vertically (like an upright mushroom) however in the photographs linked to your thread are all mounted horizontally. I am going to test the theory tomorrow and will update. Mike.