A 15W-40 would be fine as long as it doesn't get too cold. In cold weather, the 10W-30 T5 or 5W-40 T6 would be better.
The gallons are cheaper per quart than the quarts are, making it more cost effective in the end. Plus, if you buy 3 gallons by July 9 (next Friday), you can get a $10 mail in rebate on T5, or $20 on T6.
For the future after the rebate, I'd just buy 2 gallons for the first change. Next change, you'll have 1/2 a gallon already around (also available for top-off if needed), so you only need to buy 1 gallon.