Couple of things.
It could be your Hydro booster on your braking system.
You will see that you do not have a vacuum driven brake booster in your car, instead you have a hydrualic booster that mounts onto you master cylinder in the right hand corner of your engine, that is a very common leak on the M60s', and a leak there will appear as if it is coming from the rear of the engine.
Yes, you understand correctly, I am saying that power steering fluid powers your braking system, its called the H31 hydro booster brake system, and its a pain in the ass...
If you say it is when you add power steering fluid that the leak becomes more visible, then that rules out the 3 seals on your gearbox.
Does your power steering pump make a noise ?
Find your ABS pump, look towards the firewalland you will see the brake master and the hydrobooster bolted to each other, it should be about 40 mm long or so, with a triangle in the midde of it with three bolts, right there is where your leak should be.
Check it out, let me know.