My car has had only a few bottles of fuel system cleaners over 130K miles so far. I do not experience noticeable drop in engine power. I kept up with regular dyno oil changes every 3000 miles until 90K miles and 5000 miles OCI since then with synthetic. I'm not interested in any sorts of performance gain. I just want to make sure the engine is clean. Is it really necessary to run the fuel system cleaner every now and then? What is your best and favorite fuel system cleaner? I saw a post where every 3000 mile treatment is recommended. Where those deposits/residues go about? engine oil? exhaust to catalytic converter?