Rob, do a search on this as you will get almost as many opinions as there are posters.
When I bought my E34 I initially put semi-synthetic into it because I didn't know any better, nor what was in it before.
I then started using Mobil 1 5W-40. No leaks at all !
I finally located Mobil 1 0W-40, but it can be like rocking horse $hit in Oz so you may have to order it somewhere.
If you read the blurb on the back of the containers, it would appear that the 0W-40 has extended life compared to the 5W-40.
I am not suggesting to extend the period between changes, just suggesting that the 0w-40 would appear to be a superior oil.
Interestingly I have finally located a source of fully synthetic Mobil Delvac diesel engine oil for my F 250.
It is also worth looking at Mobil's websites for more info.
BigKriss' oil post from a few weeks back might be useful :
http://www.chris-longhurst.com/carbi...oil_bible.html