I disagree with your statement regarding the introduction of synthetic oil to a "high-mileage" engine. It is a misconception that synthetic oils create oil leaks, as they contain the same additives as conventional oils to maintain integrity of engine gaskets. I personally began using Mobil-1 5W30 in my CSVT at 105K miles and immediately noticed a difference in the engine. It now runs smoother and noticeably quieter, particularly at start-up, and has developed zero leaks of any kind and is going strong at 136K+ miles. Bring on the synthetics - they rule!!
