Hi Roberto1444,
Well probably you're right about the thread on non US bikes
, I'll leave that to the admin to decide.
Well in your weather a 20W50 oil would do great, I'm using a 10W40 from Ipone called Ipone 10.4 with Jaso MA, API SG. It does get cold around here and usually I ride all year long, so cold starts are quite often.
Back to what you asked, I first found a wikipedia article on that (
http://en.wikipedia.org/wiki/Motor_oil), if you read through it you'll find a section on API rating, you'll find a example of SG oil.
But I do also know that Wikipedia is not a source of 100% accurate information, so I went on searching and found a couple more things on the topic, but what really made up my mind was a pdf file I found from Zplus (
http://www.zddplus.com/TechBrief14%20-%20ZDDP%20and%20Pre-WWII%20Engines.pdf), wish is on of those ZDDP additives, that has a table with the exact information I was looking for, it's sort of a history of ZDDP and somewhat commercial, but I was happy with the finding.
I found it much easier to find API SG rated oils that having to worry about ZDDP additives...
Hope this bit of info helps.
PS- here's a MOBIL table containing ZDDP levels is some of their oil
http://www.mobiloil.com/USA-English/MotorOil/Files/Mobil_1_Product_Guide.pdf