Bonneville oil consumption

I have a 1993 Bonneville with supercharge. The engine now has over 310,000 km. The car is still running great and feel great on the road. I took the
car out for a long highway and mountain road (over 1100 km). Each time I stop for gas, I checked for engine oil level. I have noticed the oil went down slightly. Top up to a full mark with 10W30 oil that took about 3/4 of the 1 litter bottle. Drive again for 700 km more, I had to top it up to a full mark again with the rest of the oil that is 1 litter for 1800 km.
I decided to replace the PCV valve locate on top of my supercharger unit with Canadian Tire part as it is the only store around. ( I kept the old part just incase). Immediately I smell a strong gas or odour of unburned fuel somewhere under the vehicle when I stop at the rest stop (about 15 min city drive). I drove for another 1300 km for the remaining of the trip with the new PCV valve and the odour slowly gone, the oil consumption drop to almost 3/4 of a litter (little more than half of litter). My exhaust tail pipes is black but no oil and the rear bumper is clean and there is no visible residue of oil along the rear bumper either. I would expect my exhaust tail pipes to have a tan deposit in color not black after a long trip like this.
I have a few questions: My exhaust tail pipes deposit should not have black but tan in color right? does this mean that my car is start to burn oil? It passed E-test about 6 months ago.
Why did it smell gas or unburned fuel odour when I changed the PCV valve? The odour is gone now. Should I expect a car problem soon?
Any ideas?
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Just because a part is new, doesn't mean it's any good. Shake the valve to be sure the plunger rattles, if in doubt, replace valve. Oil leaks are also a factor, a tiny leak can add up to serious loss.
I recommend cleaning engine top and bottom using either a pressure washer, or a spray on hose off product, I use Castrol Super Clean on filthy parts, and Simple Green for detailing. Check cleaned engine a couple days later, a leak will be obvious. If your obsessed, remove flywheel cover, clean and reinstall.
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Thanks for the tip. This afternoon I had a closer look at the engine bottom as I am doing the engine oil change, the oil pan is a bit wet (few drops near and along the drain bolt). I am going to clean the engine from engine valve covers to the oil pan and will monitor it closely. For the black carbon deposit in my tail pipes, I am going to remove the spark plugs to check for its condition again. I have replaced the AC Delco Platinum plugs (not ignition wires) about 7 months ago and during that time, the plugs look fine. I have decided to recall the plugs from my tool box to exaimed the color of the electrode again. There is a tan in color in the center of the electrode. Spark plug number 2 and 4 has the lightness tan color while 1 and 6 has brownish color and 3 and 5 are slightly darker than other 4 plugs. Ironically, the electrode from the spark plugs color match in pair as described above if that means something. I may check for engine compression (I said may be because I am not sure if I can do it on my own as I need to get a gauge and possible tips to perform this test so I won't damage the engine ignition system as I am told).

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Today, after driving in a long bumper to bumper traffic, as I slowly accelerate with the traffic, there is an engine knocking sound like. As the traffic speed up, I can accelerate hard, the knocking is going away. Even if I stop the car then accelerate hard again, the engine knocking would not come back. I decided to remove and check each spark plugs for its condition as well as tip color. All spark plugs have light tan in color from the center of the electrode to the top of the ground pin.

octane)?
Any ideas?
P.S. My oil consumption is due to rear valve cover gasket failure. I am going to replace it and hopefully that is the only leaking source.

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