Been buying 0W-40 oil for years. The wife's 2003 Polaris 500HO ATV requires it. Always thought that was a little strange, but that's what the manufacturer wants.
As far as diesels being cheaper to run than gas engines, that's pure hockey. No way does the little bit of fuel mileage make up for the 20% premium you get to pay for diesel fuel. Maintenance is 3 times more expensive, at least, and is required at about the same intervals as a gas engine. Couldn't go over 7K miles on my last diesel without getting a bad report from Blackstone Labs, and the oil and filter cost me over $100 each time and I was doing the work. Now I don't pay over $32 out the door for oil and filter changes at a Ford dealer, and that includes a tire rotation. Factor in the two fuel filters, and diesels are much more expensive to run.