I've got a 1600 xflow. It blows smoke out of the rocker box filler cap. A compression test on all four cylinders show a good compression and within 10% of each other. I've had all sorts of advice but my local mechanic said it could be the oil rings. Is this a possibility? If it is, does it makes sense that the bores should be ok on the basis that the compression is ok?

I would make sure your breathers are not blocked before you pull the engine apart.