therwol Loc: USA

That fact has come out in recent years, beginning with the VW scandal, which caused the whole industry to come under scrutiny. Unfortunately the "system" is in place in many countries, and your options may be more limited if you insist on renting a gasoline powered car. My point is that there is no hassle associated with driving a Diesel car over there, either from the standpoint of fuel availability or performance. You can go with your conscience if you want.

Peterff Loc: O'er The Hills and Far Away, in Themyscira.

Sure, but although diesels are ubiquitous, no more or less so than gas / petrol. It makes no real difference from a practical point of view, perhaps slightly better mileage, but why damage the environment that you are going to visit? Diesel is equally available here, it's not an issue from an availability perspective.

DJphoto Loc: SF Bay Area

I don't know if it applies outside of the U.S., but I usually get the best rental car rates from Costco, sometimes from AAA. If you are not a member, it could be worth the cost of membership. Also, many times you will get charge for a second driver; not the case when renting through Costco. I suspect other warehouse clubs have similar deals.

therwol Loc: USA

Because if you're shopping for size/price in a rental, you may not be offered both options in what you want. Diesels never went over well in passenger cars in America. The big push was in the late 70s and early 80s. The domestic car offerings were noisy and had a terrible odor. VW has always had a niche market for them. Mercedes used to sell a lot of them. I don't see as many now. (But I rented one in the UK once and paid for a compact. That was a stupid story that's irrelevant.) Some grotesquely large pickup trucks are bought with diesel engines. I can see the point if you're using one to farm and haul big trailers, but people use them as passenger cars. To get back to the point I want to make, many Americans see the work Diesel and cringe. The driving experience with a modern car is essentially the same either way. The pollution issue is for your conscience. I don't see any effort to get them off of the road in the UK, by the way. Correct me if I'm wrong.

Peterff Loc: O'er The Hills and Far Away, in Themyscira.

Renting either is not a problem. Europe went down the 'clean diesel' route instead of the hybrid route, and now diesel isn't quite so clean.

It isn't an issue, a rental car of either type will be used anyway. The pollution issue will become an issue over time, but that is hard to do in the current climate and will take time to resolve.