I don't know about the old motors, but I changed in my little Dodge Neon about a month and a half ago. It is getting at least 2 mpg more than running the mineral oil. This pays for the little additional cost very quickly. I bought a fully synthetic oil from the Car Quest store and it was $3.67 per quart. I plan to change every 10000 miles, with a filter change at 5000 miles. I would not go back to the mineral oil, at least not on this vehicle.
I am now getting 32-35 mpg on fairly short trips, 12-20 miles. I honestly believe this little Neon would get 40 mpg on a long drive in summer driving. The winter driving always knocks a mpg or so off, and I am getting at least 2 mpg more on the same trips than I did under warmer temperatures.
