Here’s some stuff I use in my car that works and improves fuel efficiency: Marvel Mystery Oil (use 4 ozs. for every 10 gals) every fillup or every second fillup. It also has the added benefits of fighting gas line freeze and keeps your fuel injectors clean. If you mix it proportionally with your engine oil, it will help clean harmful deposits and remove sludge also (provided you also get a good oil filter), although you would only mix this with standard petroleum-based oil.
Another good idea that helps a lot of people and that I have used on my '89 Ford Escort LX (in addition to the Marvel Mystery Oil gas treatment) is Lucas Heavy Duty Oil Stabilizer (www.lucasoil.com). This will help lower the operating temperature of your engine, increase oil pressure and if you have an oil leak or you use oil (mine uses oil, although I don’t have an oil leak now), it will reduce your oil usage and condition the seals to stop oil leaks. It is especially good at reducing wear from starting your car.
Probably the best way to improve fuel efficiency in your car is to use a good synthetic oil. You would basically use a new oil filter on your car, then add engine flush, idling for 20 minutes; then drain your oil, replace the oil filter with a new one, then add the synthetic. The best around is Amsoil (www.amsoil.com). It is not only rated for 25,000 miles minimum use, but will improve your fuel efficiency. I used to use regular Pennzoil 10W-30 (which turns out to be one of the better oils), and treated each tank with STP, but only got 28 mpg on highway driving only on 2-3 fillups after the new oil change, so then I found I only got 24-25 mpg regularly. Once I started using the Amsoil full synthetic (10W-30, then 10W-40), I normally get 28-30 mpg, sometimes 32-34 mpg–driving the same way (highway). Plus, I would highly recommend the Amsoil Oil Filters, as they rate the highest of any filter, as does their synthetic oil. These filters are good for 12,500 miles (and one rep just told me they will have one good soon on the market for 25K miles) and have the best construction and filtering around. Although the oil and filters are a bit high for me, I cannot argue from the benefits I get from using them, especially on a 4-cyl, 140K mileage car like mine.