[lust4speed]

One of the tests many of us do before dropping the engine in the car is to point the rear of the engine as far down as we can while it is on the cherry picker, and let it set overnight. A properly sealed engine won't have any spots on the floor the next morning.

Both the rear main and the rear of the pan seal are never exposed to any direct pressure source, so it shouldn't matter whether the engine is running or not. There is a shot of oil being spewed from the rear bearing, but the oil slinger built into the crank between the bearing and seal should deflect the oil away from the seal. If there is a buildup of crankcase pressure when the engine is running, then this would provide some push to the oil spray to more easily exit the rear of the engine.

Now might be the best time to figure out where the oil is coming from. The 1/2" or so space between the main seal and the pan seal will either be wet or dry before the engine is fired again. If that area has oil on it, then the rear main isn't doing its job; and if that area is dry, then look to the pan seal. And if both areas look dry, then the leak might be from the valve covers not sealing well and allowing the oil to travel down the back of the engine.