Viscosity of branched vs. linear short alkanes
Viscosity is related to Van der Waals/London forces between molecules.  This in turn is related to packing and surface area.  

A branched 6-chain carbon, tumbling in solution cannot align as much of its surface area with another molecule as the linear hexane.  

At some point the larger molecules are becoming able to be 'entangled' and form structures somewhat similar to knots.  When the molecules get this large, viscosity increasing branching becomes more likely.  

Finally, when you get to true polymers, the chains are so long that branching again begins exclude other molecules from structure and again linear molecules are more likely to have higher viscosities (although for polymers, these are solution viscosities).