Failed solenoids (if lucky, dirty solenoids) is a very common problem with the N52. A faulty valvetronic is much less likely. I recently bought an e61 with the n52 that had been seriously neglected and had 180000 miles on it. Lots of sludge under the valve cover, stumbled/missed on idle slightly but not enough to throw a code. I cleaned the solenoids, changed the plugs, all coils (one was mis-matched), valve cover gasket, and flushed the engine twice with SeaFoam. Been a great daily ever since with no unusual idle, great throttle tip in, etc. Point being I don't think your issue with the code/stumbling is an oil problem. You could clean the solenoids for S's and G's, it's an easy 20 minute affair involving only a couple of 10mm bolts and a couple harnesses.

But I think you're going to have to find someone with a BMW specific code reader. I think it would speed things up tremendously. I find when working on BMW's... P codes are not very helpful in many cases. And stored codes (not active codes the ECU retains) can be equally valuable to a diagnosis.

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