The varnishing issue, in my opinion, is not as alarming as it's being made out to be. Varnishing is a common occurrence in engines, especially those with high mileage like the 2003 Hyundai Tiburon in question. It's an inevitable byproduct of the combustion process and the heat within the engine.
While it's true that excessive varnishing can be a sign of poor maintenance, it's also important to remember that the car in question has been under the care of its current owner for only seven years and was bought with an already high mileage of 160k. The varnishing could very well be a result of the maintenance habits of the previous owner.
As for the use of HPL's synthetic cleaner oil, I would argue that it's not a magic bullet for engine varnishing. It's an aid, not a cure. The expectation that it would drastically clean the engine after one use might be a bit unrealistic.
Moreover, the cost of HPL products is indeed a factor to consider. If the car is running fine and not showing any significant performance issues, continuously spending on expensive oil products might not be the most economical choice.
In conclusion, while it's important to maintain our vehicles and use quality products, it's equally important to have realistic expectations and make cost-effective decisions.
Now, if it was my vehicle, I would risk some money and buy a case of
@High Performance Lubricants PCMO 5W-40. That oil is amazing. Run it for a full OCI, and keep an eye on it. Buy a cheap borescope camera from Amazon and look under the valves cover every 2K miles. If you notice that it started removing the lacquer and varnish, replace the oil filter, top off your oil, and keep going. Ultimately, it is up to you if it's worth spending the money to clean up the neglect of the previous owner.