The following is an exchange that Performing Imports and I had over the past two days regarding Synthetic oil in the e34. They do not recommend it as the car was "not designed" for it. Apparently there is some concern about "Vanos rattle". I found the response interesting

Looking for reaction from other people. TIA

Russell Casey wrote 112104
I saw on one of the BMW Bulletin boards, perhaps Bimmerboard, that your company or technician does not recommend synthetic oil in a 95 525i. I bought the car at 72,000 miles-now 102,000. I did use Mobil 1 -20w50. Now I use Mobil 1 0w40 and have done so for about 15, 000 miles. I have changed the oil/filter about 3 times now after starting to use 0w40. By the way, the 0w40 stopped a slight rattle when the engine first started on a cold day.

I am worried now? Should I be?

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His response 11/22/04

Russell
That engine was never intended for synthetic oils, and we do not recommend them. But, if you are happy with the results you are getting - keep using the oil you like. It won't hurt the engine. If your Vanos unit starts to rattle you will need to switch back to 20/50.

Performing Imports