Almost sounds like the oil isn't draining from the top end down in to the engine - hence the low level, then later the higher level. I'm worried that the oil passages are caked and ugly. I would recommend a gentle engine flush.
So it's not an oil pressure issue? Just a oil level issue? Has anyone checked the oil pressure with a manual gauge?
Lastly - why petroleum Castrol 20W-50? This oil is way too viscous for your engine and may cause premature wear, even in southern CA, especially on cold mornings in the mountains. Volvo does not recommend this viscosity and has sent bulletins out in the past on it's use.
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http://www.fidalgo.net/~brook4/oilslubesfilters.html
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