Chevrolet Repair: Excessive oil consumption, excessive oil consumption, intake manifold gasket

Question
I own a 1996 Caprice Classic with a 5.7 350 LT-1 engine that has been consuming and fouling #8 cylinder. In the distance of 800 miles( mostly highway) the # 8 spark plug becomes oil fouled. The check engine light came on, then blinked and then approximately 4 miles later the car quit running.The shop that my car was towed to took a look at the car "read "the car codes and then to told me that the engine needed to be rebuilt. I asking why and they stated they had done a leak-down test and found #8 cylinders' valves were leaking at 30%. They also did a leak-down test on #6 cylinder and found it to be leaking at28%.I found this hard to believe because the engine ran so well( No blue smoke, no rough idle, no hesitation at any speed).I didn't committ to do any work so the shop move the car outside.I payed them for the 1 hour of diagnostic time (which we agreed too). By this time over two hours had past, so and I got out the tool box I carry with me and pulled the #8 plug and cleaned it and  replaced it and started the car. It idled and ran perfectly,so I drove it 150 miles back home. When I got home I put a vaccum guage on it and it produced a rock solid 18"of vacuum with no intermittent drops of the pointer. when revved to 2500-3000 rpm and then allowed to return to idle speed the vaccum drop to 0 then steadily came right back to 18" on the vaccum guage and didn't budge. I thought that if I was getting some sort of leakage in the cylinders I would see the needle of the vaccum guage  bounce or move around at idle. I hate to replace an engine for the wrong reasons or not fix the right problem. Could you head in a direction. I originally thought it might be a malfunctioing PVC valve.How far off-base am I? Thanks, Mark Richmond  

Answer
Hello Mark, The first thing to do is check the intake manifold gasket for leaks. This gasket can move and I have seen it move to the point of causing rough idle and oil consumption. To do this let the engine idle then remove the oil fill cap, did the idle change at all? If so then the gasket has moved, also there shouldn't be a lot of vacuum inside the crankcase. If this is ok then the next thing I would do is remove the right valve cover to check the drain holes for the oil return. If these holes plug up the oil can't get back to the oil pan and this can cause the oil to go down the valve stem into the cylinder. If these holes are clean it's time to replace the valve seals on that #8 cylinder.

       Let me know what you find out          Mark