Sediment in bottom of bottle?

[aquariuscsm]

I always wondered if it was a chemical reaction that occurs once the bottle is opened and exposed to air. Every new jug I've opened lately is spotless and I haven't shaken them at all,but once that half quart or so sits for a few weeks after opening the new jug to do an oil change,I'll see a very slight staining on the bottom within the grooves.

 

[steve20]

I noticed this when QSUD cam in clear bottles. About a week or so before an OC I start turning them upside down and shaking