When first running Synthetic in the BMW, there was initially a slight consumption, which cleared shortly thereafter. Valve Cover Gasket was already beginning to weep and wasn't made worse by the M1 0w-40.
Fixed it, and replaced the PCV (which when removed of crusty stuff during VCG replacement leaked like a sieve thereafter). No more issues with the synthetic.
That said, any oil that meets the appropriate API grade, and perhaps any other relevant spec is fine. I'm no longer a synthetic fan-boy, mostly because of the huge price differential here in Australia.
As for intervals, follow VW's recommendation. Some BITOG'ers would suggest a strict short-change diet, others might suggest a UOA to ensure your choice of oil will last the recommended interval or beyond.
I say stick with VW's recommendation, period, for an old vehicle. For new stuff, extending is certainly doable, but needs to be done with evidence to back it up.