Howe's, Power Service, Schaeffer's, and LCD from the top of this page make very good additives. Use the right engine oil, change the filters regularly and you'll get another 220 on the clock.
I guess I'll tack my question onto the end of this thread. What's a good brand of injector cleaner for an old diesel? My car is an '84 Volvo 240 diesel and I'm experiencing surges in power (mostly at higher RPMs) and a slight loss in fuel mileage.