I'd leave well alone, although some of the flush and drain compounds are good for cleaning out the crap in an engine. Diesels deposit soot particles here and there especially. Modern petrol engines run very very cleanly indeed.
Don't drive the engine or load it up when they're in though. Drain and refill as instructed.
For me the best thing to do is use high quality oils and stick with the same oil for topping up if your car needs this. This is very important with full synthetics.
Fuel additives are a different arsenal of beasts. Particularly Miller's and Forte's brands. Injector cleaners and the like really can help as part of a 'controlled diet'. So, avoid crap fuel, stick with the big boys, BP, Shell etc. |