Im not sure that its necessarily the mobil1 syn thats the problem, because I've had multiple turbo subarus where some would eat mobil 1 synthetic and some could go 7000 miles and maybe burn half a quart at most. For example, my legacy that I have now doesn't burn any of it. Im sure a lot of this has to do with the loose tolerances the factory has on these engines. Considering your oil level was fine though, and your crank bearings were shot, I'd lean towards some sort of knocking/detonation issues.
You'll know once you see the rest of the bearings. If they're hammered then you may have been dealing with some knock that eventually just got the best of the bearing. Or possibly the car was starved of oil at some point.
Either way, if all your internals are good, getting it back on the road with a crank polish and some new bearings isn't so bad.