In fatty acids, longer chains increase the melting point (and viscosity) by increasing London dispersion forces. Inversely, more double bonds decrease the melting point.
What happens when both chain length and number of double bonds increase at the same time; how much are London dispersion forces affected by each factor?
Eicosapentaenoic acid has a lower melting point (probably lower viscocity too) compared to docosahexaenoic and stearidonic acids. One carbon atom was added to the chain and one double bond at the same time.
18:4 -> 20:5 decrease in melting point
20:5 -> 22:6 increase in melting point
Intuitively bosseopentaenoic acid should have an even lower melting point (even lower viscosity). Conjugation would appear to make bonding between different molecules more difficult and therefore would also decrease London dispersion forces (and MP, BP, viscosity. etc with them)
The above are pictures of a space-filling model I found from google images supposedly generated from Accelrys DS Visualizer.