You have no worries. I drove mine with duct tape sealing the hole for a few months. As you noticed, the larger the crack, the more your engine bogs down and idles roughly. Just keep the hole sealed until you get a new part and you'll be good to go.
Oil, I stick with the cheap stuff. Our engines are nowhere near high performance (or high horsepower lol) that would demand synthetic.
And are you selling your 3?
Wow, you sound like an upstanding citizen.
My question is... why would you even try to weld it? Just put some duct tape over the hole, and fill it with oil. It'll run like new... hahahaha