I took a 1 day lubrication / oil seminar and they told us YES new oil in sealed containers may separate. Open containers risk increases, older oil increases, and storage poorly increases settling. The more expensive oils (more additives) also increases risk.
I tend to buy OEM oils (think expensive) and was told the more expensive the oil the higher likelihood it has higher levels of additives and subject to more settling. Higher priced (Better??) quality oils sometimes have between 10 and 30% additives and these additives can and may separate over time in sealed containers.
We talked about a huge list of additives that may be present in the oils like anti-oxidants, rust inhibitors, viscosity index improvers, anti-wear agents, ZDDP, TCP, detergents, dispersants, anti-foaming agents, friction modifiers, demulsifiers, emulsifiers, biocides, viscosity improvers, pour point depressors, etc / etc / etc..... We were told the more expensive oils have more additives and may have more settling over time.
The diesel engine oil i am buying spec'ed out to 25% additives in it and the hydraulic oil 10% so I was told in the testing part of the class to consider separation especially on drums and 5 gallon pails that have been opened (partially used). I store indoors out of heat and sun light and rain, etc... in a very moderate temp. region so risk is low but still to take care I was told. If i would switch to the cheapest oils (very few additives) then settling would be even less of a concern but then other concerns pop up.