I contacted a well known Guzzi dealer here in the US, Harper Moto Guuzi, a few months ago, got a couple of oil filters and moly additive for the final drive .
I asked about oil, told him I use a BMW branded 20w50 oil by Spectro Oils in my airhead and oilhead bikes, it's a semi synthetic blend and the comment I got, was that would be more than sufficient, just don't use auto oil .