Slow Oil leak or burning?

I have a 1988 Mazda 929 with a 3.0L Fl V6 engine. It's an automatic and has 222,996 original miles. I just bought the car (which had a new engine put in) early this year. I put 10W-30 Synthetic Blend it in and it takes 4.5 quarts to fill. It's been about a 1.5 months since I changed the oil. I just recently checked it and it was.5 quarts low. I do an average amount of driving, not too much or not too little. When I first bought the car it was running very strange as it would shake terribly when accelerating and got terrible gas mileage. I bought the car 300 miles from me and brought it back here under these conditions. After trying to replace the spark plugs a few weeks after the purchase, I noticed that one of the spark plug wires wasn't plugged in and that it was running on 5 cylinders the entire time. I wasn't sure how long the previous owner had ran it like this, but I thought that was very idiotic. After plugging it in, the car began to run much better. Could these have caused the oil leak? And could it have caused any other problems?