Yea the carbon buildup is the biggest problem with them, along with the hpfp's going bad quickly from how much work it's doing. When the 2.0T came out the hpfp's were quickly failing since it was all still a new concept.

The only problem would be me adding non-synthetic, if it even still exist lol.

Are you sure it is the rear main seal? If not, it could be a valve cover leak. There are a ton of threads about best oil to use. Choose one that is readily available and meets factory specs. There are so many you can look decide by price, whatever give you warm fuzzies or even the prettiest container. Best to stay in the weight range as recommended by BMW unless you are in an extremely cold climate. That said, if you go to a heavier weight range it might slow down the amount of oil leaking. Since it gets so cold where you live I wouldn't do that in the winter. I understand your circumstances but it is simply a bad idea to attempt to solve the problem with an additive. Much better idea to deal with the problem by adding oil when needed and going to a carwash every once in a while that sprays the under carriage.

99% sure. Valve cover gaskets were leaking but I already replaced those over the summer so they're good to go. I've been planning on dropping the shield to clean it and see where the leak is really coming from but I'm pretty damn positive it's the rear main.

Another thing that backs that up is if I drive it somewhat hard, the leak becomes bigger/quicker after parking it. If I drive like a grandmother then the leak is slower. So of course I try my best to take it easy but it's hard lol. I'm not really an asshole driver I just like to get to places quickly yet safe.

It sucks either way because I just can't get away from having cars that leak oil! '98 Grand Cherokee 5.9, '91 Integra GS-R, '89 Volvo 740. I'm 32 & have had about 10 cars and the only one that didn't have type of leak was the Passat

Since source of oil leaks can be so misleading I always get the vehicle up on jack stands and clean the engine top and bottom. Once that is done I let the engine idle up to operating temp and then with my handy dandy super bright focused beam flashlight lie underneath in wait for the highest source of fresh oil to appear. Sometimes I have had to run the RPMs up to 3000 or so and hold it for a minute or two when it hasn't shown up at an idle after 10 minutes or so. If the leak is significant enough that repair should be done that process almost always confirms the source.

NAPA sells UV engine oil dye for about $10. Dump it in and follow the same process you use to check for leaks, but using a UV flashlight ($8 at Amazon). The leak is easy to find/trace as the dye glows.