1989 Other Subaru Models Exhaust

I have a 1989 subaru GL. The car was putting out lots of smoke through the exhaust in the back. I took the car to a mechanic, and they said it might be the fuel injector because the engine oil has gasoline in it. The mechanic said the fuel injector might put out too much fuel and the car can't burn all of it. We followed the mechanic advice and replaced the fuel injector. However, the problems still persist. What other reason might be the cause for the exhaust putting out lots of smoke?