I did the unthinkable for a male, at least according to my wife, and read my manual. Says a 5-50 Synthetic or 10-50 Synthetic but no higher than 10 on the minimum (cold) rating. Suspect the filter can't pass enough heavy oil when cold is why they want you to stay with a cold rating no more than 10. Plenty of people do use inexpensive Delo 15/40 without problems though. If you didn't ride when it is really cold, I can't see the problem with the 15 on the bottom end. Of course Husaberg doesn't know what you are going to do when they make recomendations (you may live in Alaska and ride in snow) , so they cover all bases and say 10. The hot rating of 50 applies to the motor's requirements at operating temps. The 40 must be ok at operating temps, as plenty of people us the 15/40 without problems. I personally use Mobil 1 15/50 here in Arizona. I like the 50 on the top end because it gets so hot here.
Whatever you choose, do not use any automotive oils like 10-30 in it. New Automotive oils with a hot rating of 30 or less are limited in the amount of anti wear additives (800PPM) because they plug up Catalytic Converters. They also have slick additives to make them "Energy Conserving" which can make your clutch slip. Many of the Automotive 10-40 oils are formulated the same as 10-30 for simplicity and should be avoided as well. The heavy racing oils like 20-50 in the Auto section don't usually adhere to the above specs and would be ok if you are in a hot climate. Diesel oils also don't have limitations and are loaded with Anti wear additives and are highly refined as well. These are usually 15/40 grades although Shell has a 5/40 Synthetic. Of course motorcycle specific oils in the appropriate weight are ok.
The Auto racing and the diesel oils are class II meaning they are refined well to keep the parafin content to a minimum. Shell 5/40 Synthetic is a class III meaning it is a mineral oil refined so well it behaves as a Synthetic. (Our gov. says it is ok to call an oil Synthetic if it behaves as such
) Mobil 1 is a class IV which is part (mostly) class III and part class V. Expensive oils like Motul and Redline are all Synthetic and fall into a Class V. In general, the higher the class the more temperature stable. This is a benefit because to be "multi grade" the oil must be either very temp stable or run additives called polymers. (These are molecules that ball up when cold but extend out their legs like spiders when hot) Polymers do nothing for lubrication and get sheared apart in the transmission. This is less an issue if you change the oil frequently. True Synthetics also tend to keep an Engine cleaner if you participate in extended drain intervals. Again this is less important if you change the oil like you should (5 hours or so in a Berg). Synthetics also take a bit higher temps, although if things are correct so your motor temps are normal, I doubt if that plays too much into account either. On the negitive side, I find that class V Synthetics don't allow my starter sprague to catch hold very well. Hope I didn't bore anyone too much.
Dan