I heard the same thing, synthetic oil is a big no no, 10-40w mineral oil is what you should be using, i use Castrol Activ4, its 15-50w thats close enough for me, or you could go Castrol Power 1 GPS which is 10-50w part synthetic
You definitely should not run one of these bikes in with synthetic oil - and even after run-in, a synthetic oil won't provide any benefit with this type of engine. (and some claim it may even do harm).
I buy Castrol Activ 4 from Woolies for $9 a litre and that does fine.
You'll gain a lot more by changing your oil often - rather than spending big bucks on exotics.
I'm now up to around 1200kms on my Hummer. I've changed oil over 10 times during that period.
The first few oil changes had a lot of suspended particles - which were from the bike wearing in.
As time has gone on, the oil has got cleaner at every change and I can now service the oil a little less often.
The running in period is critical though and I bet dollars that changing oil often up front will increase the life of the engine by heaps.