If you're worried about engine oil and fuel system cleaners. Check out bobistheoilguy.com
The forums there will provide you with all you need to know about oils, oil testing, fuel system additives etc.
Personally, I'm going to be running some AutoRx shortly and probably Neutra as well.
If you change your dino oil every 3000K miles (5000K km) or every 5000K miles (7500K km) with a top brand synthetic. You'll be more than covered. If you're driving a Honda, be worried about North American Honda oil filters as they are simply floss. Insist that they use the Euro spec or bring your own in. If your driving a late model Toyota - check to see it is one of the sludge monsters. Any way, you'll be lost in oil there for hours and hours!
For Dino oil, go either with Pennzoil or Havoline.
For Synthetics - go with either Mobile 1 SuperSyn (improved profile over the TriSyn), Royal Purple, Shaeffar's, Amsoil, Redline or Castrol SLX (German made not US). Change it every 5000K miles and you'll be more then covered. If you're really paranoid as I am now getting about oil as well detailing, run some AutoRx all the time and Neutra in the fuel and you'll be far away covered.
Paco