Scotland are the third seeds in Pool B behind England and Argentina, but beat the Pumas 2-0 in South America in 2010.
Andy Robinson's men meet Romania on Saturday in Invercargill, with McDowall eyeing an unexpected victory for The Oaks, who next meet the Pumas, then England and Georgia in their final fixture.
He said: "From Romania's perspective the first three matches get harder, harder and the hardest is the third.
"I think potentially they (Argentina) could be more challenging for us, whereas Scotland tend to play a bit more of a physical game which tends to suit us.
"The last World Cup we saw a few teams not perform so well, especially in the first match.
"If they get us on our good day and we get them on their bad day - it would be nice to have an upset."
Like Scotland, Romania will on Thursday name their team for Saturday's match at Rugby Park Stadium.
Historically The Oaks have relied upon their forward pack and McDowall, who was a prop in the World Cup-winning All Blacks team 24 years ago, admitted up front remains key.
He added: "Romania has a lot of big guys and they do play a very physical game. Scotland does as well.
"They play it fast, they play it hard. It's something we've been looking at.
"We've been trying to prepare a game that's competitive up front and when we get our opportunities we want to spin it wide as well."
Scotland's scrum has appeared vulnerable of late and Romania hope to take advantage.
McDowall added: "It's an opportunity, but we know they'll be working on it.
"We've been working on our set play. We want to be able to hold our own ball and get good set ball.
"It certainly is a focus for us to do well there."
Romania's World Cup preparations were hit when wing Catalin Fercu withdrew from the squad, his phobia of flying proving too much to make the journey from Europe, with Adrian Apostol, who has one previous cap to his name, brought in.
McDowall denied it was a distraction.
He added: "We were all disappointed that he couldn't make the flight - the fear of flying was too much for him.
"We respect his decision and we move on. One door closes but another door opens. Adrian Apostol is coming through very young, but what a great opportunity for him."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.