Surprised to hear that your engine oil was black when it came out, it must have been in there for a long time. The oil is usually still nice and clean when it is changed I find.
To change the Bevel Box oil there are a set of photos here:
http://www.talkmorgan.com/ubbthreads.php/topics/180189/3. I would go with what Bunny says on page 3 particularly. You don't need to remove the bulkhead panels. The BB should take approx 330cc of oil to fill it to the correct level hole, over filling will cause it to run hotter and possibly leak through the seals. I use Redline Heavy Shockproof 75W/140 (75W250), others use Millars EP140, MMC say Morris Lodexol XFS 80W140 synthetic. I usually replace the BB oil at the same time as the engine oil, every 3000 miles. The BB runs hot and the oil has a hard life.
