Once again, lots of good possibilities have been listed above. If you trust the quality of the machine shop that did the work, start by doing a little testing just to see what happens. As B-man said, a single layer valley cover is for race cars. Believe me if Pontiac could have saved 10 cents in production by using a single layer cover they would have done so. I would block off the valley PCV system and just go open breathers just to experiment. If the problem goes away, put a stock style lifter cover on and your done. If the problem stays the same, then go deeper. As mentioned, ported heads can allow oil into the intake stream from multiple places. Valve cover bolt hole break through, rocker studs, push rod bulges to name just a few. Finally excessive blow-by from improper cylinder wall to ring interface is a possibility. I would think any competent machine shop could achieve at least a decent ring seal, but anything is possible I guess. Call me an old school dinosaur, but I still use a 4-step plateau honing process on the vast majority of hone jobs I perform. The grit count and amount of removal varies with the exact ring pack. For the Ross pistons you mention from Butler with a channel molly ring pack, my process is: Hone to .003" with 70 Grit stones. Hone to .001" with 220 grit stones. Hone to finish size with 280 grit stones. Then 4 strokes with a 400 stone set at 20% load. Rings will be fully seated in 15 minutes with this process.
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