Thanks Jerry. That could be a reason.
The rear bearings are very difficult to get right. And easy to get too loose. There must be little brake shoe drag, or none. I have started to adjust the bearing with a wheel on, to give much more leverage on the hub assembly when ensuring the inner nut is correctly tight and then backing off.
While the LHS one appeared to have scored the axle housing, the RHS one had not. Both failed identically ie equal amounts of lost oil. I expect both sides bearings were adjusted well enough and equally so its still a mystery. Or they need to be tighter than factory for neoprene seals.
Also its easy to get the seal in slightly eccentric- this will cause it to rotate slightly offset and possibly pump oil.
Anyway I found NOS leather seals and the problem appears to be fixed.
