Designing fuel oil piping

I'm piping a new tank for my oil furnace, and I've got a few questions:
1) This is a two-line furnace. How is the second (return) fuel line used? Does the furnace constantly recirc the fuel while running? Why?
2) This is near Portland, OR. Typical lows in the winter get down to mid 30's F. I have maybe a 50' run of pipe from the tank to the furnace, most of it outside/above ground. Given the price of copper, what's the minimum diameter tubing I can get away with? Can I use smaller diameter for the return?
Thanks,
Kelly
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The pump operates at a constant speed and pressure. Recirculating hte oil allows the pump to run at ist optimum flow and assues a good supply of oil to the burner. You don't want the pump to run dry or to constantly run against a dead head.

What does the pump manufacturer recommend? Follow their su ggestions. Saving $20 on copper may end up costing you a new pump in a year or two. This is not a place to skimp.
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