Thanks for the information Steve, all useful stuff to know. I will be interested to hear what you find with the different oil levels. I am reluctant to over fill the BB because of the possibility of leaks from the seals, as you mention. It may well cause higher temperatures with the increased internal friction. I know more about helicopter gearboxes than cars and overfilling them usually leads to high temperatures. I have tried to improve cooling airflow to my BB with a NACA duct directing air to the pinion area, not sure if it has made much difference as it still screams after 20 minutes at motorway speeds. Soundproofing the bulkhead made a big difference but thats not a good engineering solution.
While your BB is on the bench it may be worth replacing the pinion seal. The one fitted as standard is a Nitrile type and doesn't have a high temperature classification, only about 80c from memory. Several pinion seals have leaked. A higher temperature class (200c) and better quality is a Viton R23 double lip seal (55x42x8mm) but check the size before ordering a new one as I am told there are two different sizes possible. I have replaced one pinion seal by marking the nut position before removal to get the same position on refit, it worked well. The nut is a LH thread.
