Thought I'd start a fresh thread on this canister issue.
James, if you are seeing an oil pressure light when the engine is running, you may have an oil pump problem, not a canister problem. I don't know exactly where the pickup for the pump is but the uphill thing has me baffled.
Two things can go wrong with the pump - the screen on the pickup could be blocked partially, causing it to be sensitive to angle and secondly, the relief valve in the body could be getting stuck open. This would mean that you'd see the light at idle or at certain orientations of the car.
The valve at 12 o'clock in the canister is the overpressure valve, designed to open if the filter gets choked. Rarely comes into use unless the oil is filthy. This one is unlikely to fail. The 3 o'clock feature you noticed is the oil shunt pathway. That deep pocket is supposed to be the lowest point in the canister. The little hole that goes horizontally from the bottom of the pocket is the port that opens when you pull the center bolt out. Oil flows through that hole and back into the block, allowing the canister to drain. The o-ring on the bolt shuts off the shunt.
The 6 o'clock feature is actually the check valve! The valve body is down deep in that hole with a spring on top of it with the four-fingered thing you see acting as the retainer. The valve is difficult to move because it open upwards under oil pressure.
The problem is that the valve can sometimes seat correctly, resulting in the intermittent problem you and others see. It's a bear to diagnose but once you understand the design of the canister, it becomes very clear.
John - your change to Mobil 1 from Castrol could have an effect because the Mobil will flow better than the Castrol when cold. A marginal check valve would allow the Mobil to drain out faster.
I would say the problem is your canister, not the oil.
Bruno, Roger - My new canister indeed failed in seven months. The dealer has replaced it and it seems fine now. Did I just get a "bad" canister? Who knows? I'm going to watch this one like a hawk.
As for a used canister - I bought two from a really good salvage yard (double oh two Salvage in CA) and guess what, both had defective check valves! One had the valve come apart and that's when I got a good look at the design and decided that a new one was the best option. After all, if the engine has miles on it, the valve will wear.
Hope this helps somewhat,