The guy that had it before me told me to put 20/50W oil in it, so when I changed the oil/filter, thats what I put in, everybody says thats way too much, any opinions on that? Could that cause the back fiering?

way too heavy unless the engine is junk(or leaksreally bad) 20 50 is for diesels, use 10w30. oil won't cause the backfire

So can that be part of the smoke because its too heavy? And what about regular drive and over drive?

the smoke is the oil burning more than likely, teh thicker oil is less likely to burn, but synthetic is the least likely.Leave teh trans in overdrive.If it still blows smoke real bad after getting everything changed then the engine probably needs a rebuild