Mr. Montero is traveling a total distance of 600 miles + 600 miles = <<600+600=1200>>1200 miles.
There are 1200 miles / 400 miles = <<1200/400=3>>3 sets of 400 miles in 1200 miles.
So Mr. Montero needs 20 gallons/set x 3 sets = <<20*3=60>>60 gallons in all.
Since his car has 8 gallons already, then he needs to add 60 gallons - 8 gallons = <<60-8=52>>52 gallons more.
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