Here are a few oil weights, & their respective temperature ranges:
10W30 (for temp. ranges of -5°F to 85°F)
10W40 (for temp. ranges of -5°F to 115°F)
5W40 (for temp. ranges of 0°F to 115°F)
15W40 (for temp. ranges of 10°F to 115°F)
20W50 (for temp. ranges of 10°F to 115°F)
The type of oil you use in your Brozz will greatly vary, depending on your location, brand preference, & what types of mods you've done to your bike. The main thing to be aware of is that your bike is equipped with something called a wet clutch, which means that your clutch plates are immersed in your engine's oil. This is important to note, as non-motorcycle oils contain many anti-friction additives which will cause your bike's clutch to slip. When deciding on the type of oil you want to use for your bike, make sure it is either JASO or JASO-II rated (which will often also state that it is "wet-clutch approved").
Another important factor in deciding what type of oil to use is that the Brozz 250 is an air-cooled engine. This means that your engine relies on you continually cruising down the road to keep it cool; but an often-overlooked factor is making sure that you are in a gear that keeps your RPM's low (esp. for long rides), as well as using the right type / weight of oil. As a general rule, the thicker your oil viscosity is at operating temperature, the more it will help keep your engine temp down. In addition to this, a thicker oil does a better job of "sticking" to your engine components after the bike is turned off; meaning that, when you first fire your baby up in the morning, a thicker oil will help reduce the amount of wear dealt to your engine from "dry starting" it because it won't run off of your engine components as quickly as thinner oils tend to do. This said, for the benefit of helping to keep air-cooled engine temps down, & reducing the wear of "dry starts", we recommend a weight of 5W30 / 40 /or 50 for those in colder climates, & a weight of 20W40 /or 50 for those in warmer climates.
One other thing to consider is adding your own "oil additive". Lucas makes a very nice motorcycle (JASO-rated) oil additive that will not only help reduce engine wear (while running, & on cold starts), but some report it even helps reduce engine vibration (at high speeds) & smooths shifting.