I was plagued with what I thought was a weeping rear crank oil seal. I formed the view that it was due the vacuum remaining after the engine was shut down pulling the seal inwards. Well it wasn`t before you blame the rear crank oil seal do a simple test, stand the dry sump tank on the bench leave its pipes connected but set them pointing verticle and fill the bell tank with oil and leave it to stand for a couple of days, my problem was a porous casting and it weeped from one of the through holes for the bolts that attach it to the gearbox. The belltank had to be ultrasealed to cure the problem. I must add that I had replaced the crank oil seal twice before finding the true fault and tried running with the engine vented and sealed. If you remove the sump pan its possible to have a fairly good look at the crank rear oil seal and determine if the oil is coming from that area, if its leaking the back of the flywheel will be covered with oil.