Engine flush chemicals are a double-edged sword. They can have the benefit of loosening deposits, but those deposits have to go somewhere and many oil galleries in the engine are very, very small and easily blocked. Block a gallery leading to main bearing journals, and you can say good bye to THAT engine.

If you're going to go ahead with it, I would recommend the following:

Buy yourself enough oil for 3 or 4 changes (doesn't have to be high quality, just as long as it meets basic specs for that engine) and the same number of filters - they're cheap enough after all.

With the engine hot, drain the engine oil and change the filter. Refill the oil, get the engine hot again, and use your flushing compound. Follow it's instructions for how long to run the engine at idle etc, before draining the oil and refilling.

Once refilled, treat the engine very gently and run it for around 1/4 to 1/2 an hour. After that, drain the oil and change the filter again. Then go for a drive. Again, treat it gently, but give it a good run for up to an hour. Then home for another drain and filter change.

Why so many oil changes? You want to remove as much of the flushing compound as possible, as well as giving the engine the best opportunity to "cough up" the sludge etc that you are trying to remove. And once it does, you don't want that stuff circulating in your engine for any longer that necessary.

Hope the info helps mate.

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Cheers,

Anthony