We do it all the time with the GP1200 and XL760. We try to add the oil first. When we are not sure how much fuel we need, we will add the amount of oil for what we know we need, then fuel, then top off the oil. That is to say, we might put 10 gallons worth of oil, and when were done we've added 12 gallons of fuel, we add 2 more gallons worth of oil. Most important thing is when you are filling a large tank like the sit downs you don't want to add the fuel and then not have room for the oil. Also what BK said about the mixing.