Typically the A3 rated oils are blended near the high end of the 30 weight range (which is 9.3 - 12.5 cSt), so your typical A3 rated 5W-30 (30 weight) will usually be ~ 12.0 cSt.
The typical SM or A1/A5 rated 5W-30 oil will be a little thinner in viscosity usually ~ 10.5 cSt.
In a cooler climate like here in Alberta, running the little thinner 5W-30's would be fine (especially in the cooler months, fall, winter, spring). In a hotter climate like where you are, and in the Southern US, I would probably want the little thicker A3 type oil.
Since you are considering oils outside the A3, I think a good option would be to run one of the 10W-30 HDEO's (Heavy Duty Engine Oil), these oils are dual rated for both gasoline and diesel vehicles so they are fine for gas engines. These HDEO 10W-30 oils are blended up around 12.0 cSt just like the A3 rated oils.
Delo 400 LE 10W-30 … 12.0 cSt
Rotella T 10W-30 … … 12.0 cSt
These HDEO are very high quality Group II based oils and should run 5k miles no problem in your Mini. A much less expensive option compared to running a synthetic oil 5k. Here in Canada, Petro-canada makes a HDEO 10W-30 (Duron) -- and they state in the product data sheet that it may be used in cars specifying A3/B3 or A3/B4.