First, there are other qualities of oil besides the weight...and evidently, according to people who know far more than me, the other specs bmw gives for our cars matter too. Look in your manual...there are other specs (forget all the names) but it's not just weight.
Also, there's a benefit that having a long life oil in the car for a long time that you do not get with non-synthetic, or even with synthetic of incorrect spec changed more often coz you're old school and believe in the 3000 mile change interval of old.
There's a film that builds up over time...not if you change frequently.
There's also, I've heard, an issue with heat capacity. As oil breaks down, it develops a greater capacity to pull heat from engine. So like everything, there are trade offs. But me, I think my car is pretty amazing as designed and am quite sure the BMW engineers thought a lot about what's the best oil for it...going thicker to decrease leaking is putting the cart ahead of the horse. The oil thickness isn't causing the leak or curing it. You've got a leak, or your burning more than you should...fix it and use the right oil
BTW, my 200K mile 328i doesn't burn any oil...and only dripped about 3 drops/day at most. Now, after my fixing it over a long three-day diy...better not drip anything...
But, drips are good...they tell you you need to fix something!