Things just keep looking better and better for this engine. 4.0 Jeep engine...oil changed after 500 miles because the former owner abused the hell out of it. Oil is black, but that is to be expected, and I can tell the engine is getting cleaner. Now I can tell the oil smells pretty gassy...after only 500 miles. I know this is often from the rings/cylinders being worn out, but are there any other causes? Maybe a problem with the PCV system?