[Sven7]

A lot of talk about oil here, and not much about anything else. Just change it at the manufacturer's recommended interval. I use Valvoline Max-Life 5W-30 although am not sure it is any better than normal oil for high mileage engines. Wix or NAPA Gold filters...

As far as vehicle selection, I would be wary about composite bodied cars. As Saab Sonett owners will tell you, although the body may look clean, the frame is still just as exposed as every other car on the road. You want to have the whole car protected, not just what's visible.

Apparently Porsche galvanized their car bodies too, but I would not recommend a Porsche in this challenge.

Perhaps a Volkswagen diesel would do. Sure, VW's have their issues but by doing your own work and getting a car that was really taken care of, you can easily get over 300,000 miles while doing over 50 mpg. A Golf or Jetta will have very good aftermarket support as well.

I would go for a MK4 VW TDI or a 6th gen Honda Civic. For the Civic you'd want to get a clean car from a non-salt state, with an impeccable service record. Get the entire thing professionally undercoated as 2000neon has done. From there you can replace things as they break and even have a "new" engine built for when, after a few hundred thousand miles, your original one blows up. Parts are cheap.

Do think about aero-mods and 5th gear swaps, too. With a boat tail you can actually feel how little throttle you need to use compared with stock. Of course that's going to be better for the engine! I imagine coasting in neutral will be, too. If you get an 88-00 Honda be very nice to the transmission because they don't like roughhousing.