[nkaminar]

As others have suggested, the plug gaskets are leaking. The engine is pumping oil past the rings so if the engine is not newly rebuilt then the rings are worn. A knock means the bearings need tightening. If you are not mechanically competent, then you either need to study up or find someone who is. My advice would be to take the engine out of the car and work on it on an engine stand. The work can be done in the car but it is easier if you can just turn the engine over to work on the bearings. Take the head off and see if you can move the pistons in the cylinders when they are near the top. If you can then you need an overhaul. If not then you can get by with new rings. The bearings can be tightened up by taking one or more shims out. Of course the cylinders and crank journals have to be examined for scoring.

The oil may also be coming from worn valve guides allowing oil to suck up through the guides. While you have the head off you can check this by moving the valves when they are up. If you can move them then you need new guides and maybe valves.

If the journals look worn you will have to take the crank out to measure them. If out of round or tapered than the crank will have to be reground.

While you are at it check everything else, like the clutch and cam gear.

Good luck.